[FRAME] Warn on unchecked weight witness (#1818)

Adds a warning to FRAME pallets when a function argument that starts
with `_` is used in the weight formula.
This is in most cases an error since the weight witness needs to be
checked.

Example:

```rust
#[pallet::call_index(0)]
#[pallet::weight(T::SystemWeightInfo::remark(_remark.len() as u32))]
pub fn remark(_origin: OriginFor<T>, _remark: Vec<u8>) -> DispatchResultWithPostInfo {
	Ok(().into())
}
```

Produces this warning:

```pre
warning: use of deprecated constant `pallet::warnings::UncheckedWeightWitness_0::_w`: 
                 It is deprecated to not check weight witness data.
                 Please instead ensure that all witness data for weight calculation is checked before usage.
         
                 For more info see:
                     <https://github.com/paritytech/polkadot-sdk/pull/1818>
   --> substrate/frame/system/src/lib.rs:424:40
    |
424 |         pub fn remark(_origin: OriginFor<T>, _remark: Vec<u8>) -> DispatchResultWithPostInfo {
    |                                              ^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default
```

Can be suppressed like this, since in this case it is legit:

```rust
#[pallet::call_index(0)]
#[pallet::weight(T::SystemWeightInfo::remark(remark.len() as u32))]
pub fn remark(_origin: OriginFor<T>, remark: Vec<u8>) -> DispatchResultWithPostInfo {
	let _ = remark; // We dont need to check the weight witness.
	Ok(().into())
}
```

Changes:
- Add warning on uncheded weight witness
- Respect `subkeys` limit in `System::kill_prefix`
- Fix HRMP pallet and other warnings
- Update`proc_macro_warning` dependency
- Delete random folder `substrate/src/src` 🙈 
- Adding Prdoc

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

substrate/frame/support/test/tests/pallet.rs

@@ -210,12 +210,13 @@ pub mod pallet {
 	{
 		/// Doc comment put in metadata
 		#[pallet::call_index(0)]
-		#[pallet::weight(Weight::from_parts(*_foo as u64, 0))]
+		#[pallet::weight(Weight::from_parts(*foo as u64, 0))]
 		pub fn foo(
 			origin: OriginFor<T>,
-			#[pallet::compact] _foo: u32,
+			#[pallet::compact] foo: u32,
 			_bar: u32,
 		) -> DispatchResultWithPostInfo {
+			let _ = foo;
 			let _ = T::AccountId::from(SomeType1); // Test for where clause
 			let _ = T::AccountId::from(SomeType3); // Test for where clause
 			let _ = origin;