Strengthen backwards-compatibility CI checks

[ianbotsf]

Now that we've gone 1.0/GA and we've had some time with the binary compatibility validator, we should strengthen the checking we do to monitor backwards compatibility. Specifically, we should:

• Re-enable BCV scans on @InternalApi and @InternalSdkApi symbols
• Add a CI check which requires only forward-compatible API diffs, bypassable with a new PR tag
• Review our version policy for correctness and detail about how/when backwards-incompatible changes are made and how that affects versioning and upgrades

[github-actions]

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