docs: move release notes from 0.20.0 to 0.21.0

[starius]

Change Description

In #10352 release notes were added to 0.20 by mistake.

Remove the chain notifier RPC note from the 0.20 release notes and add it to 0.21.

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
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[saubyk]

Ack