Cumulus: skip building a block on relay parents in old session

[sandreim]

Collators building on older relay parents must skip building blocks when the chosen RP session is different than best block session.

The relay chain will not accept such candidate. We can see this happening on the Kusama Canary parachain at each session boundary:

2025-10-08 13:28:06.515 DEBUG tokio-runtime-worker parachain::collator-protocol: [Relaychain] Collation wasn't advertised because it was built on a relay chain block that is now part of an old session. candidate_hash=0xf81e6266a4f125e0ef19b34d5d45844eab88359c77bbe016c547ee35ceb1f997 pov_hash=0xf9ae84c7f5750d1421f7e55357226854e285ba6182d066256680daa0569c7c4c traceID=329806307668875987237751301795633005646