Add lucene-backward-codecs to the server classpath

[AntonEbel]

Motivation

Reading a Lucene index/segment written by an older Lucene major version requires
lucene-backward-codecs on the classpath. Concretely, OpenSearch 2.x writes Lucene 9
segments (codec Lucene912), and reading those with the server's Lucene 10 (lucene-core
ships only Lucene104) fails with:

org.apache.lucene.index.IndexFormatTooOldException: Could not load codec 'Lucene912'.
An SPI class of type org.apache.lucene.codecs.Codec with name 'Lucene912' does not exist.
The current classpath supports the following names: [Lucene104].

This must live on the server classpath, next to lucene-core: Lucene's Codec SPI
registry is bound to lucene-core's class loader. Providing lucene-backward-codecs only
from a plugin (loaded in an isolated child class loader) leaves the older codecs invisible
to the SPI at read time — the plugin's copy is never resolved.

Approach

Add lucene-backward-codecs (managed at ${lucene.version}) as a dependency of
graylog2-server, alongside the existing lucene-core/lucene-analysis-common
dependencies, so it is shaded into graylog.jar and available to the codec SPI for every
consumer of the server classpath.

This is a prerequisite for the OpenSearch-snapshot archiving feature (Graylog2/graylog-plugin-enterprise#14922),
which reads OS 2.x (Lucene 9) snapshot segments directly with Lucene. That feature previously declared
the dependency in its own plugin, where the SPI could not see it.

/nocl

[fpetersen-gl]

Nice and easy. Thanks.