Release 2.15.0.0

[lukas-vlcek]

OpenSearch 2.15.0 has been released, see release notes. Official OpenSearch release ticket: https://github.com/opensearch-project/opensearch-build/issues/4681

Checklist:

• [x] Compatibility Matrix is updated (https://github.com/Aiven-Open/prometheus-exporter-plugin-for-opensearch/pull/283)
• [x] Release 2.15.0.0 tag is present in the repo
• [x] Release artifacts are available and downloadable
• [x] Release notes published
• [x] Forum announcement published

[lukas-vlcek]

Release done, waiting for release https://github.com/Aiven-Open/prometheus-exporter-plugin-for-opensearch/issues/279 to make a combined forum announcement.

[lukas-vlcek]

Forum announcement: https://forum.opensearch.org/t/prometheus-exporter-plugin-2-15-0-0-and-1-3-17-0-released/20020