ci(wordpress): record plugin releases as GitHub releases

[bendersej]

Background

WordPress plugin releases go through WordPress.org SVN, so GitHub has no record of them. The WordPress release workflow now leaves one: a wordpress@<version> GitHub release, which also marks the version as announced.

Changes

• The WordPress release workflow creates a wordpress@<version> GitHub release on the commit it runs from, with the README.txt changelog entries
• The release is never marked Latest, so the badge stays on the SDK packages
• A version whose GitHub release already exists is skipped, so running the workflow twice never posts twice on Discord
• Order: live on WordPress.org, not yet released, Discord post, GitHub release; a failed Discord post leaves no release, so a rerun is safe
• The release steps in wordpress/README.md describe it