[CI/Release] Pipeline to create a stable release branch (#5598)

This PR contains a pipeline which is going to branch off the new stable
release branch (e.g. `stab2412`, `stable2503`) and bump `polkadot`
`NODE_VERSION`, `spec_version` of the runtimes and reorganisation of the
`prdocs` related to the new stable release.

This is a first step in the automated `polkadot-sdk` release flow as
part of the task: https://github.com/paritytech/polkadot-sdk/issues/3291

The pipeline is not supposed to be triggered in the main` polkadot-sdk`
repo, but in the fork in the
[`paritytech-release`](https://github.com/paritytech-release/polkadot-sdk)
org, where the whole release flow is going to land.

Closes: https://github.com/paritytech/release-engineering/issues/222