[meta] strict on-runtime-upgrade CI checks to prevent accidental runtime upgrade breakages

[liamaharon]

try-runtime-cli on-runtime-upgrade has stabilised over the last few months to that point where we can add CI checks to ensure Polkadot & Substrate PRs don't accidentally break upcoming polkadot/kusama/rococo/westend runtime upgrades

Creating a meta issue to track this progress.

Remaining TODOs:

• Fix existing migration issues
  • Fix Society v2 migration substrate#14421
  • fix migrations (also companion for #14421)  polkadot#7454
• Update polkadot CI to run on-runtime-upgrade on every polkadot PR ci: always run runtime migration checks polkadot#7422
• Merge migrations: VersionedRuntimeUpgrade substrate#14311
• Implement VersionedRuntimeUpgrade for Unreleased pallet migrations
• Add a check to substrate CI that will fail if a companion needs to be written to prevent runtime migration breakage ci: check runtime migrations substrate#14618
• Fix Polkadot HostConfiguration: https://polkadot.polkassembly.io/referenda/9

After some time has passed and we are confident in the robustness of the CI checks:

• Block merging until on-runtime-upgrade checks are green https://github.com/paritytech/ci_cd/issues/785

[liamaharon]

Done