Migrate to a different Maven publishing plugin

[snazy]

Nessie and CEL-java are using the Gradle Nexus publish plugin, which works great for publishing to Nexus and Sonatype OSSRH. Sonatype OSSRH reaches EOL on June 30, 2025 - a migration to "Maven Central Repository" is necessary. Sonatype mentions that is's enough to update the configuration of the gradle-nexus/publish-plugin. A "canary" release of CEL-Java worked fine, but a Nessie release ran into quite some errors (empty deployment, missing mandatory files). This might be because of the Portal OSSRH Staging API, which is a partial reimplementation of the OSSRH / Nexus Repository Manager 2 Staging APIs. The noteable differences between CEL-Java and Nessie is that Nessie has way more artifacts and the publishing/deployment takes much longer (< 5 minutes vs 25-30 minutes).

In the mid/long term it is likely anyway the better alternative to use the Portal Publisher API. Althought there are many Gradle plugins around that support that API, none seems to fully fit the needs of multi-project deployments - to publish an aggregated file containing the contents for all modules to be released. Two Gradle plugins looked promising though:

• GradleUp/nmcp (author also took over the shadow plugin) has support for "aggregated deployments", but in the current form the plugin fails for projects that change the build-directory, which is what the Nessie Spark extensions do.
• zenhelix/maven-central-publish however was chosen as it was easier to integrate, but required some work in our root build.gradle.kts integrating the support for aggregated multi-project deployments.