Remove STIXGeneral from the font stack (#10980)

* Remove STIXGeneral from the font stack

STIXGeneral was originally added to our font stack to work around a bug in Chrome (https://bugs.chromium.org/p/chromium/issues/detail?id=591346) which caused some obscure combining marks to render as tofu. However, because STIXGeneral unexpectedly has glyphs for a handful of common Japanese characters, it's ended up making Japanese text in Element look patchy. I previously attempted to fix this by prioritizing sans-serif over STIXGeneral, but as is evident from our screenshot tests and user reports, this is still not enough on some systems to get Chrome to pick a consistent font for Japanese. On the basis that i18n is more important than supporting a few mathematical diacritics, I propose we remove the font. STIXGeneral is deprecated anyways, so if we want to get these diacritics back there's always the option of looking at its successor, STIXTwo.

* Remove STIXGeneral installation from Cypress workflow

.github/workflows/cypress.yaml

@@ -120,12 +120,6 @@ jobs:
             - uses: browser-actions/setup-chrome@c485fa3bab6be59dce18dbc18ef6ab7cbc8ff5f1
             - run: echo "BROWSER_PATH=$(which chrome)" >> $GITHUB_ENV
 
-            - uses: tecolicom/actions-use-apt-tools@ceaf289fdbc6169fd2406a0f0365a584ffba003b # v1
-              with:
-                  # Our test suite includes some screenshot tests with unusual diacritics, which are
-                  # supposed to be covered by STIXGeneral.
-                  tools: fonts-stix
-
             # There's a 'download artifact' action, but it hasn't been updated for the workflow_run action
             # (https://github.com/actions/download-artifact/issues/60) so instead we get this mess:
             - name: 📥 Download artifact

res/themes/legacy-light/css/_legacy-light.pcss

@@ -1,23 +1,17 @@
 /* Nunito lacks combining diacritics, so these will fall through
    to the next font.  Helevetica's diacritics sometimes do not combine
    nicely (on OSX, at least) and result in a huge horizontal mess.
-   Arial empirically gets it right, hence prioritising Arial here.
+   Arial empirically gets it right, hence prioritising Arial here. */
-   We also include STIXGeneral explicitly to support a wider range
-   of combining diacritics (Chrome fails without it, as per
-   https://bugs.chromium.org/p/chromium/issues/detail?id=1328898).
-   We should never actively *prefer* STIXGeneral over the default font though,
-   since it looks pretty rough and implements some non-LGC scripts only
-   partially, making, for example, Japanese text look patchy and sad. */
 /* We fall through to Twemoji for emoji rather than falling through
    to native Emoji fonts (if any) to ensure cross-browser consistency */
 /* Noto Color Emoji contains digits, in fixed-width, therefore causing
    digits in flowed text to stand out.
    TODO: Consider putting all emoji fonts to the end rather than the front. */
 $font-family: "Nunito", "Twemoji", "Apple Color Emoji", "Segoe UI Emoji", "Arial", "Helvetica", sans-serif,
-    "STIXGeneral", "Noto Color Emoji";
+    "Noto Color Emoji";
 
 $monospace-font-family: "Inconsolata", "Twemoji", "Apple Color Emoji", "Segoe UI Emoji", "Courier", monospace,
-    "STIXGeneral", "Noto Color Emoji";
+    "Noto Color Emoji";
 
 /* unified palette */
 /* try to use these colors when possible */

res/themes/light/css/_light.pcss

@@ -1,23 +1,17 @@
 /* Nunito and Inter lacks combining diacritics, so these will fall through
    to the next font. Helevetica's diacritics sometimes do not combine
    nicely (on OSX, at least) and result in a huge horizontal mess.
-   Arial empirically gets it right, hence prioritising Arial here.
+   Arial empirically gets it right, hence prioritising Arial here. */
-   We also include STIXGeneral explicitly to support a wider range
-   of combining diacritics (Chrome fails without it, as per
-   https://bugs.chromium.org/p/chromium/issues/detail?id=1328898).
-   We should never actively *prefer* STIXGeneral over the default font though,
-   since it looks pretty rough and implements some non-LGC scripts only
-   partially, making, for example, Japanese text look patchy and sad. */
 /* We fall through to Twemoji for emoji rather than falling through
    to native Emoji fonts (if any) to ensure cross-browser consistency */
 /* Noto Color Emoji contains digits, in fixed-width, therefore causing
    digits in flowed text to stand out.
    TODO: Consider putting all emoji fonts to the end rather than the front. */
-$font-family: "Inter", "Twemoji", "Apple Color Emoji", "Segoe UI Emoji", "Arial", "Helvetica", sans-serif, "STIXGeneral",
+$font-family: "Inter", "Twemoji", "Apple Color Emoji", "Segoe UI Emoji", "Arial", "Helvetica", sans-serif,
     "Noto Color Emoji";
 
 $monospace-font-family: "Inconsolata", "Twemoji", "Apple Color Emoji", "Segoe UI Emoji", "Courier", monospace,
-    "STIXGeneral", "Noto Color Emoji";
+    "Noto Color Emoji";
 
 /* Colors from Figma Compound https://www.figma.com/file/X4XTH9iS2KGJ2wFKDqkyed/Compound?node-id=559%3A120 */
 /* ******************** */