Add a manual release workflow, test on OTP 28 and 29

[asabil]

Replaces publish.yml, which ran when a release was published, with a manual release.yml.

Run it from main with a version such as 0.2.0 and the name and email of the author of the release commits. It:

1. checks the version and that the tag v0.2.0 does not exist
2. sets {vsn, "0.2.0"} in src/sqlc.app.src and runs the unit tests
3. commits "Release 0.2.0" and checks the package with rebar3 hex publish --dry-run
4. pushes the commit to main and creates the release v0.2.0 for it, which also creates the tag
5. publishes to Hex
6. commits "Post release version bump", which sets {vsn, git} again

Publishing needs the HEX_API_KEY secret, as before.

CI now tests OTP 29.1 and 28.5 with rebar3 3.27.1 and drops 27.1 and 26.2.5. actions/checkout moves from v3 to v7. erlef/setup-beam stays on v1, which is its latest major.

--dry-run --yes was run locally against a copy of the repository, and it validates the package and builds the docs without publishing. The workflow itself has not been run.