chore(cells): Add @deprecated to IssueGroupActionEndpoint

[kcons]

This is consistent with our policy on plugins, and the plans for cellularization, and gives remaining users are clearer indication and a clear timeline.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/plugins/bases/issue2.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #104604      +/-   ##
===========================================
- Coverage   80.64%    80.53%   -0.12%     
===========================================
  Files        9335      9330       -5     
  Lines      403011    400761    -2250     
  Branches    25695     25695              
===========================================
- Hits       325001    322735    -2266     
- Misses      77544     77560      +16     
  Partials      466       466              

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀