Make highlighting limits stricter

Currently, whenever we highlight text via `Gitlab::Highlight` as
rich text, it takes a long time (~3s). It falls back to plain text
highlighting after 3 seoncds due to the limit set via
`TIMEOUT_FOREGROUND`.

We are reducing that to 1.5 seconds so we don't have to wait that
long before falling back to plain highlighting.

We are also reducing `MAXIMUM_TEXT_HIGHLIGHT_SIZE` to 512 KB so
we don't need to wait 1.5 seconds before falling back as well when
highlighting a huge file.

changelogs/unreleased/224506-reduce-highlight-limits.yml

+---
+title: Make highlighting limits stricter
+merge_request: 39934
+author:
+type: performance

lib/gitlab/highlight.rb

@@ -3,8 +3,8 @@
 module Gitlab
   class Highlight
     TIMEOUT_BACKGROUND = 30.seconds
-    TIMEOUT_FOREGROUND = 3.seconds
-    MAXIMUM_TEXT_HIGHLIGHT_SIZE = 1.megabyte
+    TIMEOUT_FOREGROUND = 1.5.seconds
+    MAXIMUM_TEXT_HIGHLIGHT_SIZE = 512.kilobytes
 
     def self.highlight(blob_name, blob_content, language: nil, plain: false)
       new(blob_name, blob_content, language: language)