Prepare 5.2.1 release and fix publication regressions

[p4535992]

This PR prepares a possible 5.2.1 maintenance release with a deliberately limited scope.

Main changes:

• Fix Gradle publication dependency scopes reported in Class file for org.jose4j.lang.JoseException not found #219 and [BUG] All dependencies suddenly have scope "runtime" in 5.1.2 #233.
• Add a regression check for the Maven metadata generated by Gradle.
• Preserve non-default ports in the VAPID audience origin, addressing Notification::getOrigin Does not handle non standard ports leading to incorrect VAPID JWT aud. #153.
• Add regression coverage for the unpadded Base64 URL encoding fix related to Crypto-Key header: illegal format #216.
• Update the internal Gradle/Sonatype publishing implementation while keeping the existing release workflow compatible.
• Make scripts/version.sh portable across environments.
• Update the changelog with the fixes currently present after 5.1.2.

No major dependency upgrades or broad refactoring are included in this PR.

There is already a 5.2.0 tag in the repository, but it does not appear to have been published to Maven Central. For that reason, this PR currently targets a possible 5.2.1 maintenance release rather than modifying or reusing the existing 5.2.0 tag.

I am waiting for confirmation from the project author/maintainer before proceeding with the actual release, tagging, Maven Central publication, or closing related issues.

Feedback on the intended release version and scope is welcome before taking any release action.