Fix publishing of spotless-maven-plugin to include maven-metadata.xml

[mnk]

The maven-metadata.xml file was not being published to Maven Central for the Spotless Maven Plugin. This updates the publishing configuration in gradle/java-publish.gradle to use the mavenPlugin software component (provided by the maven-plugin-development Gradle plugin) instead of the plain java component. This ensures the necessary plugin descriptor and maven-metadata.xml are generated and included during publication.

I am not able to test this, so not sure if it actually works.

Fixes #520

[nedtwigg]

Here's Claude's take on this fwiw:

The blocker — the PR's core change is fundamentally unworkable, not just
merge-broken:

• from components.mavenPlugin fails at configuration: mavenPlugin is the
  gradlex DSL extension, not a SoftwareComponent. I decompiled gradlex
  maven-plugin-development 1.0.3 (the pinned, latest version) — it applies
  JavaPlugin and attaches the plugin descriptor to components.java. It never
  registers a mavenPlugin component.
• gradlex doesn't support plugin-prefix / G-level metadata at all — that's an
  open, unimplemented upstream issue (gradlex Apply IntelliJ-suggested Javadoc issue fixes. #43
  (Support plugin prefix mapping gradlex-org/maven-plugin-development#43)).
• The PR's real goal (Maven can't locate this plugin due to "org.apache.maven.plugin.prefix.NoPluginFoundForPrefixException: No plugin found for prefix 'spotless'" #520 (Maven can't locate this plugin due to "org.apache.maven.plugin.prefix.NoPluginFoundForPrefixException: No plugin found for prefix 'spotless'" #520))
  needs a group-level com/diffplug/spotless/maven-metadata.xml containing
  spotless. Neither maven-publish nor the gradle-nexus plugin
  generates this — it requires a custom generate + upload into the Sonatype
  staging repo step, and it can't be validated without a real release.