[PROPOSAL] Cleanup unused branches

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What/Why

What are you proposing?

Cleanup unused branches across client repos.

What users have asked for this feature?

When we look at various vulnerability dashboards we look at 2.x branches. Most clients no longer ship anything out of 2.x.

What problems are you trying to solve?

One have active branches to avoid people being worried about security vulnerabilities in inactive ones.

• Cleanup unused branches opensearch-go#547
• Cleanup unused branches opensearch-java#986
• Cleanup unused branches opensearch-net#645
• Cleanup unused branches opensearch-php#191
• Cleanup unused branches opensearch-py#744
• Cleanup unused branches opensearch-ruby#252
• Cleanup unused branches opensearch-rs#257
• Cleanup unused branches opensearch-js#775

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Done for language clients.