fix(unmerge): Add unmerge PUT endpoint

[jangjodi]

Unmerge endpoint is DELETE instead of PUT, causing it to use DELETE permissions
This is causing members to not have permission to unmerge issues

This PR adds the PUT endpoint, while keeping the DELETE endpoint until the FE call uses the new PUT endpoint
FE change: #79125

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/issues/endpoints/group_hashes.py	60.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #79122      +/-   ##
==========================================
+ Coverage   78.27%   78.30%   +0.02%     
==========================================
  Files        7137     7134       -3     
  Lines      314561   314310     -251     
  Branches    51359    51331      -28     
==========================================
- Hits       246226   246110     -116     
+ Misses      61885    61752     -133     
+ Partials     6450     6448       -2