Update supported browser versions

This increases supported browser versions to:

- Mozilla Firefox: at least major version 91, as this is the latest ESR
  and it is still used quite a lot.
- Google Chrome/Chromium/Microsoft Edge: at least major version 92
  because it is about the same age as Firefox ESR 91
- Apple Safari: at least version 14.1 (dropping support for
  Safari 13 and 13.1) because it is the newest minor in the previous
  major version of Safari

This is still more permissive than what we strive to support, but still
strikes a good balance:

https://docs.gitlab.com/ee/install/requirements.html#supported-web-browsers

Changelog: changed

.browserslistrc

@@ -3,14 +3,13 @@
 # https://docs.gitlab.com/ee/install/requirements.html#supported-web-browsers
 # with the following reasoning:
 #
-# - We should support the latest ESR of Firefox: 78, because it used quite a lot.
-# - We use Edge/Chrome >= 84 because 83 had an annoying bug which would mean we
-#   need to polyfill Array.reduce: https://bugs.chromium.org/p/chromium/issues/detail?id=1049982
-# - Safari 13.1 because it is the current minor version of the previous major version
+# - We should support the latest ESR of Firefox: 91, because it used quite a lot.
+# - We use Edge/Chrome >= 92 because they are about as old as the Firefox ESR
+# - Safari 14.1 because it is the current minor version of the previous major version
 #
 # See also this epic: https://gitlab.com/groups/gitlab-org/-/epics/3957
 #
-chrome >= 84
-edge >= 84
-firefox >= 78
-safari >= 13.1
+chrome >= 92
+edge >= 92
+firefox >= 91
+safari >= 14.1
\ No newline at end of file