Release v1.5.0 (the Plaza upgrade on Kusama)

[seadanda]

I'm posting early to propose a release for Contracts and the Asset Hub migration on Kusama in April or early May, to organise ourselves in advance. Hopefully we can co-ordinate the update and get the reviews so we can make a release soon after the release of stable2503.

• Upgrade to polkadot sdk stable2412 #552
• Update to stable2503
• Integrate contracts functionality on Kusama (MVP, excluding precompiles, which will be released in a later version)
• Add pallets to be migrated to AH to the asset-hub-kusama runtime
• Introduce the migration pallet (developing on branch https://github.com/polkadot-fellows/runtimes/tree/dev-asset-hub-migration)

Feel free to add features and fixes below in the comments, but let's keep the primary aim of the release for the Plaza upgrade and prioritise accordingly.