Add support for Elasticsearch 8.x

[indigo423]

The plugin can only be compiled against Elasticsearch 7.x. Using Elasticsearch 8.x breaks with the following error message:

[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  9.161 s
[INFO] Finished at: 2024-07-23T15:40:54+02:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal on project elasticsearch-drift-plugin-8.14.3: Could not resolve dependencies for project org.opennms.elasticsearch:elasticsearch-drift-plugin-8.14.3:jar:2.0.5-SNAPSHOT
[ERROR] dependency: org.elasticsearch.plugin:aggs-matrix-stats-client:jar:8.14.3 (provided)
[ERROR]         Could not find artifact org.elasticsearch.plugin:aggs-matrix-stats-client:jar:8.14.3 in central (https://repo.maven.apache.org/maven2)
[ERROR]
[ERROR] -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/DependencyResolutionException

We use the dependency aggs-matrix-stats-client which doesn't exist for 8.x versions.

[mmahacek]

This is important as Elasticsearch 7.x is only supported until 9.0 is released. Plus v8.x has been around for quite a while.