docs: Update changelog - drop unreleased header for 4.3.0

[michalbundyra]

Add new API for single-call flag evaluation in flags.

💡 Motivation and Context

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Added the release label to the PR
• Added exactly one version bump label: bump-patch, bump-minor, or bump-major

[marandaneto]

thanks @michalbundyra