Deprecate frame_system::block_number

[gupnik]

With the addition of BlockNumberProvider to all pallets as being tracked in #3268, we should deprecate frame_system::block_number as suggested by @kianenigma in #5723 (comment)

[kianenigma]

Reading my comment in #5723 (comment) again, I wonder if a unified type BlockNumber: trait BlockNumberProvider

with

trait BlockNumberProvider {
    fn current_block_number() -> u32;
    /// None if solochain, Some(_) if para. 
    fn relay_block_number() -> Option<u32>;
}

Is acceptable? This would remove the need to alter every single pallet.

we're def leaking the idea of "relay" chain to all solo chains, but our aim is to make polkadot-sdk parachain-first anyways.