Set git attributes to union changes in the CHANGELOG to reduce merge conflicts

[cwperks]

Description

This is a quality of life PR to drastically reduce the number of merge conflicts and prevent maintainers from having to constantly resolve conflicts only in the CHANGELOG file.

By setting the gitattribute merge=union, this tells git to automatically take the union of both sides of the conflicts without maintainers having to manually determine how to perform the merge.

Related Issues

Resolves opensearch-project/OpenSearch#18475

Related to https://about.gitlab.com/blog/gitlab-reduced-merge-conflicts-by-90-percent-with-changelog-placeholders/

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.18%. Comparing base (69916f4) to head (9bdebb8).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5384      +/-   ##
==========================================
+ Coverage   72.15%   72.18%   +0.02%     
==========================================
  Files         382      382              
  Lines       23697    23697              
  Branches     3644     3644              
==========================================
+ Hits        17099    17106       +7     
+ Misses       4804     4795       -9     
- Partials     1794     1796       +2     

see 1 file with indirect coverage changes

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