Move major tag update (e.g. v6) into the release workflow as a
conditional step, gated on semantic-release actually publishing.
This avoids the GITHUB_TOKEN limitation where events from the
default token don't trigger separate workflows.
Also reverts @semantic-release/github from .releaserc since we no
longer rely on the release event to trigger a separate workflow.
The update-major-tag workflow triggers on `release: published`, but
semantic-release was only creating git tags (via @semantic-release/git)
without creating actual GitHub Release objects. This meant the workflow
never fired.
