refactor!: move latestTag logic to ReleasePR

[chingor13]

Delegates the logic of finding the latestTag to the individual releaser. In order to support the stable branch names, we need to leverage the detection of the version from the merged pull request.

This is breaking as it moves the implementation of GitHub#latestTag to ReleasePR.

Fixes #756

[codecov]

Codecov Report

Merging #758 (59b9c87) into master (f41a777) will decrease coverage by 0.77%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #758      +/-   ##
==========================================
- Coverage   90.95%   90.17%   -0.78%     
==========================================
  Files          57       57              
  Lines        7426     7515      +89     
  Branches      719      720       +1     
==========================================
+ Hits         6754     6777      +23     
- Misses        671      737      +66     
  Partials        1        1              

Impacted Files	Coverage Δ
src/github.ts	83.75% <ø> (-3.69%)	⬇️
src/releasers/ruby.ts	35.00% <0.00%> (ø)
src/release-pr.ts	90.78% <85.08%> (-1.32%)	⬇️
src/releasers/go-yoshi.ts	92.96% <100.00%> (ø)
src/releasers/java-bom.ts	92.88% <100.00%> (ø)
src/releasers/java-yoshi.ts	87.31% <100.00%> (ø)
src/releasers/node.ts	100.00% <100.00%> (ø)
src/releasers/ocaml.ts	94.11% <100.00%> (ø)
src/releasers/php-yoshi.ts	92.99% <100.00%> (ø)
src/releasers/python.ts	100.00% <100.00%> (ø)
... and 4 more

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