Move coverage to Java 11

[bitwiseman]

Description

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We love getting PRs, but we hate asking people for the same basic changes every time.

• Push your changes to a branch other than main. Create your PR from that branch.
• Add JavaDocs and other comments
• Write tests that run and pass in CI. See CONTRIBUTING.md for details on how to capture snapshot data.
• Run mvn -D enable-ci clean install site locally. If this command doesn't succeed, your change will not pass CI.

When creating a PR:

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[codecov]

Codecov Report

Merging #1299 (ccf8685) into main (50d4ae6) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #1299      +/-   ##
============================================
- Coverage     77.89%   77.86%   -0.04%     
  Complexity     2011     2011              
============================================
  Files           197      197              
  Lines          6217     6220       +3     
  Branches        358      362       +4     
============================================
  Hits           4843     4843              
  Misses         1160     1160              
- Partials        214      217       +3     

Impacted Files	Coverage Δ
...ava/org/kohsuke/github/GitHubRateLimitChecker.java	69.23% <0.00%> (-5.77%)	⬇️
src/main/java/org/kohsuke/github/GHRepository.java	68.42% <0.00%> (ø)

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