feat(code-suggester)!: move to code-suggester for PR management

[bcoe]

integrate release-please with code-suggester, benefits:

1. the release PR comes in as a single commit (allowing us to stop adding [skip ci]).
2. it seems as though this stops the behavior of PRs opening and closing as they're updated.

Depends on: googleapis/code-suggester#95

[chingor13]

This looks nice!

[JustinBeckwith]

LGTM with annoying nits

[codecov]

Codecov Report

Merging #536 into master will increase coverage by 0.56%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #536      +/-   ##
==========================================
+ Coverage   81.69%   82.25%   +0.56%     
==========================================
  Files          42       42              
  Lines        4993     4819     -174     
  Branches      407      414       +7     
==========================================
- Hits         4079     3964     -115     
+ Misses        913      854      -59     
  Partials        1        1              

Impacted Files	Coverage Δ
src/github-release.ts	93.16% <ø> (-0.05%)	⬇️
src/releasers/java-bom.ts	93.70% <ø> (-0.10%)	⬇️
src/releasers/java-yoshi.ts	87.85% <ø> (-0.26%)	⬇️
src/releasers/simple.ts	92.70% <ø> (-0.08%)	⬇️
src/updaters/changelog.ts	100.00% <ø> (ø)
src/updaters/java/google-utils.ts	100.00% <ø> (ø)
src/updaters/java/java_update.ts	96.25% <ø> (-0.10%)	⬇️
src/updaters/package-json.ts	100.00% <ø> (ø)
src/updaters/php-client-version.ts	88.37% <ø> (-0.52%)	⬇️
src/updaters/php-manifest.ts	88.88% <ø> (-0.31%)	⬇️
... and 10 more

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