[Deprecation] Deprecate `decl_*` macros

[KiChjang]

We've now basically migrated from the previous function-like decl_* macros into the #[pallet] attribute macros, and it's time that we start deprecating/removing the old decl_ macros.

[yourarj]

@KiChjang I would like to take it up.

[ggwpez]

There are two merge requests leading up to this #12401 and #12445 which are both stale&closed.
Maybe someone can give this another nudge? The old macros still pollute the code base.