Migrate to com.vanniktech.maven.publish

[fornwall]

Releasing 0.12.0 failed at the final step:

Execution failed for task ':releaseSonatypeStagingRepository'
> Staging repository is not in desired state [released, not_found]: ... state=closed

io.github.gradle-nexus.publish-plugin reaches the Central Portal through the
OSSRH staging API compatibility service, which has no release endpoint, so the
deployment has to be released by hand at https://central.sonatype.com/publishing.
That plugin has also not had a release since April 2024.

com.vanniktech.maven.publish talks to the portal API directly and can release
as part of the build.

Changes

• Replaces the signing, maven-publish and io.github.gradle-nexus.publish-plugin
  plugins with com.vanniktech.maven.publish 0.37.0, which applies the first two itself.
• Drops withJavadocJar()/withSourcesJar(), as the plugin adds both.
• Replaces nexusPublishing/publishing/signing with a single mavenPublishing block.
• Updates the comment documenting how to release.

The onlyIf predicate that skips signing for publishToMavenLocal is kept: the
new plugin signs local installs otherwise, which is what #26 fixed.

Verification

• The generated POM is byte for byte identical to the one the old configuration produced.
• publishToMavenLocal installs jar, sources, javadoc, pom and module, and skips signing.
• signMavenPublication produces signatures that gpg --verify accepts.
• build passes with 23 tests and no warnings.

Not verified: the actual upload and release, which can only be exercised by a real
release. Credentials are read from mavenCentralUsername/mavenCentralPassword
rather than sonatypeUsername/sonatypePassword.

🤖 Generated with Claude Code