Enforce `MaxEncodedLen` impl for `NposSolution`

[kianenigma]

See: https://github.com/paritytech/substrate/pull/11032/files#r832354293 (#11032)

okay, so about these, we need a follow up issue to try and actually enforce this bound.

It should not be too hard, we only ever create this type in a single function, from_assignments, and in there, we need to sum up all the fields (votes1, votes2, ...) and make sure they are less than MaxVoters::len().

I will make a new issue for this so you can do it as a follow-up.