ci: Limit workflow scope branches

This commit updates the CI workflows that trigger on both push and pull
request events to limit their event trigger scope to the main and the
release branches.

This prevents these workflows from simultaneously triggering on both
push and pull request events when a pull request is created from an
upstream branch to another upstream branch (e.g. pull requests from
the backport branches to the release branches).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>