Add publishing plugin to elasticsearch-grok project (#89184)

It seems https://github.com/elastic/elasticsearch/pull/88982 introduced
a dependency on `elasticsearch-grok` to `x-pack-core`. Since the latter
is published to Maven Central, this means consumers will have issues
resolving it's dependencies since `elasticsearch-grok` isn't published.
This pull request resolves this, by adding the publishing plugin to the
`grok` library. We'll then follow up separately to add that to our
release configuration.

libs/grok/build.gradle

@@ -5,6 +5,7 @@
  * in compliance with, at your election, the Elastic License 2.0 or the Server
  * Side Public License, v 1.
  */
+apply plugin: 'elasticsearch.publish'
 
 dependencies {
   api 'org.jruby.joni:joni:2.1.29'