chore: add @InternalApi to binary compatibility checks; introduce new PR check to verify no API-breaking changes

Issue \

This change tightens backwards compatibility guarantees by:

Removing @InternalApi from the exemptions for the backwards compatibility verifier
- Also updates the API dumps because a lot of previously-exempted APIs are now in scope
Adds a PR check which verifies that either no API dump lines have been deleted or the acknowledge-api-break label is applied to the PR
- Example run with no breaking changes (succeeded)
- Example run with breaking changes, no label (failed)
- Example run with breaking changes, acknowledge-api-break label (succeeded)

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