chore: prepare release 0.21.2

[github-actions]

This PR was created automatically. Merging it will create a new release for 0.21.2

Features

• RHP4 graceful shutdown

Fixes

• Always cancel read side of QUIC stream when calling stream.Close.
• Fixed an edge case in RPCFreeSectors where the deletion logic would keep a sector root that was supposed to be deleted.
• Pass proper descriptions when closing QUIC connection.
• Fixed excessive "peer relayed a v2 header with unknown parent" log spam by only relaying headers after sync when the synced blocks became the new tip.
• Update Go version to 1.26.0.
• Update mux dependency to v1.5.0.
• Use DEBUG log level instead of ERROR for RPCs failing due to the peer closing the connection on their end.