.github/workflows: split release into pre-release-ci and release (manual)

[jaylim-crl]

Previously, we run the release workflow automatically whenever a v* tag gets pushed. The existing workflow does not work because the tests can be flaky. When that happens, there's no way to release a new TF version since goreleaser will only run once all the acceptance tests pass.

Until the flakiness goes away, we will split the release workflow into two: pre-release-ci (that runs automatically like before), and release (which has to be triggered manually after all the acceptance tests are triaged).

Commit checklist

• [X] Changelog -- irrelevant
• [X] Doc gen (make generate) -- irrelevant
• [X] Integration test(s) -- irrelevant
• [X] Acceptance test(s) -- irrelevant
• [X] Example(s) -- irrelevant

[jaylim-crl]

TFTR!