Combine Gemfile.linters into main Gemfile

Splitting out from A03-6048 as this piece proved to cause issues. Context below, but see that original issue for more details.

We shouldn't need to have 2 Gemfiles with the linting gems separate. Should be able to package them into development dependencies and have everything working "out-of-the-box". However, last attempt caused app startup issues for some (and https://github.com/otwcode/otwarchive/pull/3895 was ultimately reverted).