Backport recent commits from polkadot staging (#2864)

* Running relayer: documentation (#2829)

* running relayer documentation

* fix review comments

* also update tag

* typo

* Apply suggestions from code review

---------

Co-authored-by: Adrian Catangiu <[email protected]>

* [Do Not Merge until RBH/WBH upgrade] Relayers use new submit_finality_proof_ex call in Rococo <> Westend bridge (#2827)

* use codegen runtime in WBH + update codegen files for RBH + WBH

* use submit_finality_proof_ex for Rococo <> Westend bridge

* Document: how to make relayer releases (#2860)

* RELEASE.md

* updated running-relayer.md

* Prepare P<> K relay release: 1.2.0 (#2863)

* cargo update -p mio

* bump relayer version: v1.2.0

* Cargo.lock

---------

Co-authored-by: Adrian Catangiu <[email protected]>