docs: update release section

[carole-lavillonniere]

The "Regular release" section in README-LOCALSTACK.md predates the weekly auto-release and no longer matches how releases are cut.

Changes:

• Rename it to Manual release and move it after Weekly auto-release, since the weekly workflow is now the regular path.
• Limit manual releases to minor/major bumps and urgent fixes that can't wait for Friday.
• Warn that a pushed tag is published as a full release and skips the localstack-pro validation gate, and show how to keep the gate (gh release edit vX.Y.Z --prerelease).
• Drop the -pre tag suffix advice: the weekly version lookup and the localstack-pro promotion expect plain vX.Y.Z tags.