refactor!: Pass RepositoryMergeRequest and RepoMergeUpstreamRequest by value

[JamBalaya56562]

This continues the #3644 value-parameter refactor for the two request bodies in github/repos_merging.go.

Repositories.Merge (POST /repos/{owner}/{repo}/merges)

• Merge now takes RepositoryMergeRequest by value.
• The schema-required Base and Head become non-pointer string (dropping omitempty); the optional CommitMessage stays *string with omitempty. Matches the OpenAPI schema 1:1 (required: [base, head]).

Repositories.MergeUpstream (POST /repos/{owner}/{repo}/merge-upstream)

• MergeUpstream now takes RepoMergeUpstreamRequest by value.
• The schema-required Branch becomes a non-pointer string (required: [branch]).

Both types were removed from the paramcheck body-allowed-pointer-types allowlist in .golangci.yml, and accessors were regenerated.

Towards #3644

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.51%. Comparing base (90a1849) to head (eeb1a2f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4372   +/-   ##
=======================================
  Coverage   97.51%   97.51%           
=======================================
  Files         193      193           
  Lines       19526    19526           
=======================================
  Hits        19040    19040           
  Misses        269      269           
  Partials      217      217           

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[gmlewis]

Thank you, @JamBalaya56562!
LGTM.
If you could please add the succint "BREAKING CHANGE:" line to the description, that would be appreciated.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra