Terraform validator: document release process & release

[melinath]

Currently there is no documentation on the release process. We should add documentation, establish a formal release cadence, and potentially make a new release along the way.

The documented release process should also include tagging and release notes on Github, and start using semver (currently only date-based). This will allow us to better indicate when releases have breaking changes.

https://groups.google.com/a/google.com/g/terraform-team/c/hdJwgc4sW_E/m/bpNsJ4asBAAJ

[melinath]

Once this is done we need to also open a ticket to update in g3.

[melinath]

This is sufficiently resolved for now.

[github-actions]

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