Fix inconsistent heading style in email diffs

See also [this MR][1]

[1]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55284

app/assets/stylesheets/mailers/highlighted_diff_email.scss

@@ -27,7 +27,7 @@ $highlighted-gd-bg: #fdd;
 $highlighted-gd-x: #000;
 $highlighted-gd-x-bg: #faa;
 $highlighted-gr: #a00;
-$highlighted-gh: #999;
+$highlighted-gh: #800080;
 $highlighted-gi: #000;
 $highlighted-gi-bg: #dfd;
 $highlighted-gi-x: #000;
@@ -185,7 +185,9 @@ span.highlight_word {
 
 .ge { font-style: italic; }
 .gr { color: $highlighted-gr; }
-.gh { color: $highlighted-gh; }
+
+.gh { color: $highlighted-gh;
+  font-weight: $gl-font-weight-bold; }
 
 .gi {
   color: $highlighted-gi;

changelogs/unreleased/ps-fix-heading-style-email-diffs.yml

+---
+title: Fix inconsistent heading style in email diffs
+merge_request: 55420
+author:
+type: fixed