Add unmaintained `diff_match_patch` gem to `vendor/gems`

This MR adds the unmaintained diff_match_patch gem to vendor/gems.

The diff_match_patch gem is used generate diff patches. The diff_match_patch gem has not been updated in over 10 years and is no longer maintained. This MR adds the gem to vendor/gems so that it can continue to be used by GitLab.

How to set up and validate locally

1. Clone the GitLab repository.
2. Run bundle install.
3. Run rake test in the ./vendor/gems/diff_match_patch directory with ruby 3.1+.

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.

---

This description was generated for revision 3670ed87 using AI