Handle strings in `delete`'s `limit` field

[noisersup]

Mongodb allows delete's limit parameter to be a string. I think that adding string handling in common.GetWholeNumberParam would be sufficient, although it's used for other operators like $size so the issue would require to find usages of the function and cover all of them with string tests. We should also unskip delete's ValidString test.

[AlekSi]

We should check if that's still a case with MongoDB 6 (#1083).