elastic scaling: rework core selector handling

[alindima]

Prior to the PR, the parachain runtimes were prevented from sending the core selector ump signal by the experimental-ump-signals compile-time feature. This was needed because the validators were not yet upgraded to be able to decode the new signals.

This PR removes the experimental-ump-signals feature and makes the runtime send the UMP signal if the SelectCore impl (part of the runtime config trait) returns Some(...). This is a breaking change on the SelectCore trait so cannot be backported.
It also adds an impl for (), which returns None.

Instructions on how to configure the parachain runtime for elastic scaling will be included in #6739

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/12390057192
Failed job name: test-linux-stable