Elastic scaling: launch checklist

[sandreim]

This is the list of tasks to be completed for launching elastic scaling on Polkadot:

• [x] https://github.com/paritytech/polkadot-sdk/pull/6452
• [x] https://github.com/paritytech/polkadot-sdk/pull/6739
• [x] https://github.com/paritytech/polkadot-sdk/issues/6442
• [x] Release stable2412 which includes runtime and node/collator side implementation of RFC103
• [x] https://github.com/paritytech/polkadot-sdk/pull/7254
• [x] stable2503: Stable Slot based collator
• [x] Referenda to enable RFC103 on Kusama
• [x] Referenda to enable RFC103 on Polkadot
• [ ] Kusama/Polkadot runtime upgrade to stable2412
• [ ] Remove the experimental-ump-signals compile feature from the parachain-system pallet (making the behaviour it's guarding as the default)