Add threading for deprecation processing

Why is this change being made?

⚠ This MR needs !121197 (merged) merged first.

This MR adds threading to the deprecations array processing, which reduces the total time taken to generate the RSS feed by ~97% 🚀 :

From master-patch-2bd1 branch

$ bundle exec bin/generate-breaking-changes-rss  3.43s user 1.01s system 1% cpu 4:36.87 total

From this branch (ashmckenzie/add-threading-for-deprecation-processing)

$ bundle exec bin/generate-breaking-changes-rss  1.35s user 0.45s system 15% cpu 11.809 total

Verification

$ time bundle exec bin/generate-breaking-changes-rss
info: Connecting to GitLab...
info: Retrieving deprecation files from API...
info: Loading breaking changes...
info: Creating /Users/ash/src/gitlab/www-gitlab-com/public/breaking-changes.xml...

$ ls -la
total 608
drwxr-xr-x   4 ash  staff     128 15 Mar 16:27 .
drwxr-xr-x  50 ash  staff    1600 10 Mar 15:56 ..
-rw-r--r--   1 ash  staff  154740 15 Mar 16:26 breaking-changes.xml.orig
-rw-r--r--   1 ash  staff  154740 15 Mar 16:21 breaking-changes.xml.threaded

$ sha256sum breaking-changes.xml.orig breaking-changes.xml.threaded
ae1a7aeda14f5a289fdd51604eae5a24f008dd6050aedb494611af18e8e92917  breaking-changes.xml.orig
ae1a7aeda14f5a289fdd51604eae5a24f008dd6050aedb494611af18e8e92917  breaking-changes.xml.threaded

$ diff breaking-changes.xml.orig breaking-changes.xml.threaded | wc -l
       0

Related: !121197 (merged)

cc @lstahlman

Author Checklist

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