⚠️ Remove ListMergedPRs API

[azeemshaikh38]

• Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• PR title follows the guidelines defined in https://github.com/ossf/scorecard/blob/main/CONTRIBUTING.md#pr-process

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Instead of relying on 2 potentially divergent APIs - ListCommits and ListMergedPRs, reduce the API to only ListCommits. Any PRs associated with this commit will be return in the associatedPR field. It makes Scorecard more tightly tied to analyzing a repo's commits (part of Commit based scorecard #575) and helps avoid confusion in future implementations of RepoClient (long-term fix of BUG: Checks using ListMergedPRs() need hardening #1524).

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  Yes.

[github-actions]

Integration tests success for
[af87dc0]
(https://github.com/ossf/scorecard/actions/runs/1768084213)

[github-actions]

Integration tests success for
[5fc4c1d]
(https://github.com/ossf/scorecard/actions/runs/1768086946)

[codecov-commenter]

Codecov Report

Merging #1566 (fb3e449) into main (86d8281) will increase coverage by 0.04%.
The diff coverage is 41.07%.

@@            Coverage Diff             @@
##             main    #1566      +/-   ##
==========================================
+ Coverage   54.12%   54.17%   +0.04%     
==========================================
  Files          70       70              
  Lines        6169     6167       -2     
==========================================
+ Hits         3339     3341       +2     
+ Misses       2620     2614       -6     
- Partials      210      212       +2     

[github-actions]

Integration tests success for
[479a387]
(https://github.com/ossf/scorecard/actions/runs/1769997586)

[github-actions]

Integration tests success for
[b7fff68]
(https://github.com/ossf/scorecard/actions/runs/1775734336)

[github-actions]

Integration tests success for
[0a6d247]
(https://github.com/ossf/scorecard/actions/runs/1780250252)

[laurentsimon]

LGTM overall. Thanks

[github-actions]

Integration tests success for
[fb3e449]
(https://github.com/ossf/scorecard/actions/runs/1781613996)

[azeemshaikh38]

Friendly ping for a review.

[github-actions]

Integration tests success for
[3afdd3a]
(https://github.com/ossf/scorecard/actions/runs/1785032017)

[github-actions]

Integration tests success for
[a58ab56]
(https://github.com/ossf/scorecard/actions/runs/1786770579)

[github-actions]

Integration tests success for
[c0a8c8d]
(https://github.com/ossf/scorecard/actions/runs/1786777859)