🐛Fix overweight contribution

[JuaniRios]

Built on top of #243

What?

• The proof sizes on ProjectsToUpdate for contribution_ends_round extrinsic seems way to high. We fixed them

Why?

• We don't know for sure

How?

• I changed from a BoundedVec to a single tuple for ProjectsToUpdate, that seemed to fix it, but when I reran the benchmark on main it produced a similar result to the tuple change, so the original error might have been a mistake on the machine.

Anything Else?

• Even though the change of this PR probably wasn't the cause of the fix, I think it's worth to include since the boundedVec we use is always of length 1

[vstam1]

• Probably requires Migration if we want to update Politest network.
• Did you run all tests in pallet-funding + integration tests?
  Approved if comments are fixed

[lrazovic]

Also, let's fix the warnings before merging:

warning: variable does not need to be mutable
   --> pallets/funding/src/benchmarking.rs:571:7
    |
571 |         let mut block_number: BlockNumberFor<T> = inst.current_block() + T::EvaluationDuration::get() + One::one();
    |             ----^^^^^^^^^^^^
    |             |
    |             help: remove this `mut`
    |
    = note: `#[warn(unused_mut)]` on by default

warning: for loop over an `Option`. This is more readably written as an `if let` statement
    --> pallets/funding/src/lib.rs:1246:37
     |
1246 |             for (project_id, update_type) in ProjectsToUpdate::<T>::take(now) {
     |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: `#[warn(for_loops_over_fallibles)]` on by default
help: to check pattern in a loop use `while let`
     |
1246 |             while let Some((project_id, update_type)) = ProjectsToUpdate::<T>::take(now) {
     |             ~~~~~~~~~~~~~~~                         ~~~
help: consider using `if let` to clear intent
     |
1246 |             if let Some((project_id, update_type)) = ProjectsToUpdate::<T>::take(now) {
     |             ~~~~~~~~~~~~                         ~~~
     ```