v218 for Monday release (heroku#1034)

Release v217 with only stack image change (heroku#1029)

* Gracefully handle unrecognised stacks

Previously if the current stack was not in the download presence check's
list of known stacks, the compile would fail with:

```
lib/language_pack/helpers/download_presence.rb:32:in `next_stack': undefined method `+' for nil:NilClass (NoMethodError)
	from /app/tmp/buildpacks/.../lib/language_pack/ruby.rb:469:in `warn_stack_upgrade'
	from /app/tmp/buildpacks/.../lib/language_pack/ruby.rb:462:in `warn_outdated_ruby'
	from /app/tmp/buildpacks/.../lib/language_pack/ruby.rb:115:in `block in compile'
```

Refs W-7457678.

* v217

* [changelog skip] Fix CNB tests

Co-authored-by: Ed Morley <501702+edmorley@users.noreply.github.com>

v216 release for monday (heroku#1023)

Merge pull request heroku#978 from heroku/schneems/buildpack-cache-in-ci

Fix CI Test runs not using bundler cache

Merge pull request heroku#975 from heroku/schneems/absolute-yarn-path

Fix yarn relative path in PATH

Merge pull request heroku#973 from heroku/schneems/cd_bug_again

Fix absolute path bug in .profile.d/ruby.sh $PATH

Merge pull request heroku#972 from heroku/schneems/v212

v212

Merge pull request heroku#962 from heroku/schneems/v211

v211

Merge pull request heroku#958 from heroku/schneems/fix-logic-inversion

Fix version download warning logic

Merge pull request heroku#957 from heroku/schneems/less-agressive-war…

…ning

Make version warnings more robust