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.

[gui1117]

I wonder if we actually want to use the relay chain block number that often.

If we want a number that increments regularly, we probably actually want to use the time.
If we want a number that increments related to the chain progress then we want the chain block number.

The only reason to use the relay chain block number instead of the time would be if the relay chain don't produce block for a while, and so the parachain can't push its task on time.