pallet-membership should implement ContainsLengthBound (#4865)

### ISSUE
Link to the issue:
https://github.com/paritytech/polkadot-sdk/issues/1143



Deliverables
 - Implement trait `ContainsLengthBound` for pallet-membership
 

### Test Outcomes
___
Successful tests by running `cargo test -p pallet-membership --features
runtime-benchmarks`



running 22 tests
test tests::__construct_runtime_integrity_test::runtime_integrity_tests
... ok
test benchmark::bench_clear_prime ... ok
test tests::add_member_works ... ok
test tests::change_key_with_same_caller_as_argument_changes_nothing ...
ok
test tests::change_key_works ... ok
test benchmark::bench_set_prime ... ok
test benchmark::bench_remove_member ... ok
test benchmark::bench_change_key ... ok
test tests::change_key_works_that_does_not_change_order ... ok
test benchmark::bench_reset_members ... ok
test benchmark::bench_add_member ... ok
test tests::genesis_build_panics_with_duplicate_members - should panic
... ok
test benchmark::bench_swap_member ... ok
test tests::query_membership_works ... ok
test tests::prime_member_works ... ok
test tests::test_genesis_config_builds ... ok
test tests::remove_member_works ... ok
test tests::migration_v4 ... ok
test tests::swap_member_with_identical_arguments_changes_nothing ... ok
test tests::swap_member_works_that_does_not_change_order ... ok
test tests::swap_member_works ... ok
test tests::reset_members_works ... ok

test result: ok. 22 passed; 0 failed; 0 ignored; 0 measured; 0 filtered
out; finished in 0.01s

   Doc-tests pallet_membership

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered
out; finished in 0.00s

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Co-authored-by: Bastian Köcher <[email protected]>