Migrate pallet-proxy to pallet attribute macro (#8365)

* Migrate pallet-proxy to pallet attribute macro

Part of #7882.

Converts the `Proxy` pallet to the new pallet attribute macro introduced in #6877.

[Upgrade guidelines used](https://substrate.dev/rustdocs/v3.0.0/frame_support/attr.pallet.html#upgrade-guidelines).

## ⚠️ Breaking Change  ⚠️ 

From [checking upgrade guidelines](https://crates.parity.io/frame_support/attr.pallet.html#checking-upgrade-guidelines)

> storages now use PalletInfo for module_prefix instead of the one given to `decl_storage`: use of this pallet in `construct_runtime!` needs careful updating of the name in order to not break storage or to upgrade storage (moreover for instantiable pallet). If pallet is published, make sure to warn about this breaking change.

So users of the `Assets` pallet must be careful about the name they used in `construct_runtime!`. Hence the `runtime-migration` label, which might not be needed depending on the configuration of the `Assets` pallet.

### Notes

There are some changes to the docs in metadata for the constants. The docs in the metadata for constants are now more complete.