Fix key set order attribute_name parameter

Gitlab::Pagination::Keyset::Order top comment examples shows attribute_name as receiving a symbol,
however, if that parameter is NOT a string it won't translate cursor values returning empty result sets.

We should be able to accept attribute_name as String or Symbol

The following discussion from !50579 (merged) should be addressed:

• @felipe_artur started a discussion:

  @ahegyi I think I did catch a bug here. If attribute_name is NOT a string it won't translate cursor values always returning null.

  Opening a follow-up to investigate.