Introduce BWC testing infrastructure

[lukas-vlcek]

Description

This commit introduces necessary changes to enable BWC testing.

Closes: #315

---

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[lukas-vlcek]

Currently, this PR introduces some new WARNINGs (for java21):

> Task :bwcCluster#oldVersionClusterTask1
Dec 30, 2024 3:48:44 PM sun.util.locale.provider.LocaleProviderAdapter <clinit>
WARNING: COMPAT locale provider will be removed in a future release

> Task :bwcCluster#fullRestartClusterTask
Dec 30, 2024 3:49:06 PM sun.util.locale.provider.LocaleProviderAdapter <clinit>
WARNING: COMPAT locale provider will be removed in a future release

> Task :bwcCluster#oldVersionClusterTask0
Dec 30, 2024 3:49:29 PM sun.util.locale.provider.LocaleProviderAdapter <clinit>
WARNING: COMPAT locale provider will be removed in a future release

> Task :bwcCluster#mixedClusterTask
Dec 30, 2024 3:49:48 PM sun.util.locale.provider.LocaleProviderAdapter <clinit>
WARNING: COMPAT locale provider will be removed in a future release

> Task :bwcCluster#twoThirdsUpgradedClusterTask
Dec 30, 2024 3:50:04 PM sun.util.locale.provider.LocaleProviderAdapter <clinit>
WARNING: COMPAT locale provider will be removed in a future release

> Task :bwcCluster#rollingUpgradeClusterTask
Dec 30, 2024 3:50:17 PM sun.util.locale.provider.LocaleProviderAdapter <clinit>
WARNING: COMPAT locale provider will be removed in a future release

These will be fixed once fix like this opensearch-project/OpenSearch#11550 will be back-ported to OpenSearch 2.x branch.