Update version to 1.43.0.dev/0.64b0.dev (#5226)

* Update version to 1.43.0.dev/0.64b0.dev

* ci: skip alls-green check for version bumps in merge_group

Skip the alls-green status check when it's a merge_group action and the
commit message matches the expected commit message for the post-release
version bump PR.

Assisted-by: Gemini

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Co-authored-by: otelbot <197425009+otelbot@users.noreply.github.com>
Co-authored-by: aaronabbott <aaronabbott@google.com>