Pallets: Treasury deprecate `propose_spend` dispatchable

[muharem]

Deprecate propose_spend dispatchable and it's dependant dispatchables, reject_proposal and approve_proposal.

The spend dispatchable is expected to be used instead.

Deprecation Issue: https://github.com/paritytech/polkadot-sdk/issues/138

[juangirini]

I know that "Remove usage of the deprecated feature in the code base" is listed as a separate item in the deprecation process, but I think we should already use the new syntax instead of using#[allow(deprecated)] everywhere

[ggwpez]

I know that "Remove usage of the deprecated feature in the code base" is listed as a separate item in the deprecation process, but I think we should already use the new syntax instead of using#[allow(deprecated)] everywhere

But we have to keep the tests in tact for as long as the code is not completely removed, otherwise it could silentlybreak.
The #[allow(deprecated)] is just for the tests here.

[muharem]

but merge

[muharem]

bot merge