chore(ci): fix the changelog check

[nirinchev]

Description

The previous check was incorrect - it was never actually running because the step name was invalid. I've moved it to a dedicated workflow because we need different triggers for the check to work correctly (e.g. it wouldn't pick up manually adding a label, because that would not trigger the workflow). I've also simplified some of the logic as we were using quite a few steps before which could be combined into a single one.

Checklist

• New tests and/or benchmarks are included.
• Documentation is changed or added.
• Changelog is updated accordingly.
• I have signed the MongoDB Contributor License Agreement (https://www.mongodb.com/legal/contributor-agreement).

Open Questions

[github-actions]

Coverage Report

There is no coverage information present for the Files changed

[github-actions]

🤖 Benchmark Comparison for Don't enforce changelog for merge commits

Benchmark	Previous	Current	Change
com.mongodb.jbplugin.jmh.SampleBenchmark.init	2,499,964,015.59 ops/s	2,633,858,863.01 ops/s	5.36%

[github-actions]

🤖 Benchmark Comparison for Move changelog enforcement to a dedicated workflow

Benchmark	Previous	Current	Change
com.mongodb.jbplugin.jmh.SampleBenchmark.init	2,499,964,015.59 ops/s	2,630,951,379.72 ops/s	5.24%

[himanshusinghs]

Thanks for taking this up, looks good 🚀