internal/scms/github: allow any merge method for Github PRs

[seankhliao]

The MergeMethod parameter is optional, and already defaults to merge if left unset.
Unsetting it should allow it to work with any other merge method if merge was disabled in settings.
https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#merge-a-pull-request

Fixes #673

[codecov-commenter]

Bundle Report

Bundle size has no change ✅

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.87%. Comparing base (be6b09b) to head (1ada550).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #691      +/-   ##
==========================================
- Coverage   56.12%   55.87%   -0.25%     
==========================================
  Files          37       37              
  Lines        3854     3853       -1     
==========================================
- Hits         2163     2153      -10     
- Misses       1398     1402       +4     
- Partials      293      298       +5     

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[zachaller]

I have a feeling this will break history calculations? Although once we move no-op support to the hydrator I think this makes sense. Have you tested this and validated history still behaves?

[seankhliao]

Is that history in argocd or in the promoter?