Migrate publishing from OSSRH to Sonatype Maven Central Portal

[maxlambrecht]

PR Description

Changes

• Integrates the Vanniktech Maven Publish plugin for Central Portal publishing.
• Removes old OSSRH repository configuration.
• Updates Gradle and adjusts Javadoc/Sources JAR generation.
• Adds GitHub Actions workflow for publishing to Central Portal.
• Updates CI to drop Java 8 from the test matrix (the new publishing plugin requires at least Java 11).
  Artifacts remain Java 8–compatible; only CI execution changed.

The new setup was tested by publishing a dummy version, confirming that artifacts were successfully staged in the Sonatype Central Portal.

This aligns java-spiffe with Sonatype’s supported publishing path and ensures reliable future releases.