[stable2506] Backport #8860 relay chain logic (runtime + client)

[paritytech-release-backport-bot]

Backport #8860 into stable2506 from serban300.

See the documentation on how to use this bot.

This PR backports the relay chain client and runtime changes related to #8860.

This way we could start relaxing the advancement rule on the relay chain, and in a future release we will introduce the new parachain runtime logic.

[github-actions]

This pull request is amending an existing release. Please proceed with extreme caution,
as to not impact downstream teams that rely on the stability of it. Some things to consider:

• Backports are only for 'patch' or 'minor' changes. No 'major' or other breaking change.
• Should be a legit fix for some bug, not adding tons of new features.
• Must either be already audited or not need an audit.

Emergency Bypass

If you really need to bypass this check: add validate: false to each crate
in the Prdoc where a breaking change is introduced. This will release a new major
version of that crate and all its reverse dependencies and basically break the release.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/16268976115
Failed job name: test-linux-stable