refactor: rename fields from maximum to maximum_allowed

[milgner]

For compatibility with ActiveSupport 7 which monkey-patches Enumerable with a method called maximum.

Closes #182

[adfoster-r7]

I imagine this might impact other projects as well, as there's a high possibility that maximum is being used with bindata objects elsewhere too.

After syncing up with @cdelafuente-r7 it looks like this might be a change that we consider introducing to bindata itself to handle this scenario differently:

https://github.com/dmendel/bindata/blob/a6d900a3066e979d091ac6d0845a035b3b4ecf1a/lib/bindata/struct.rb#L64-L78

[adfoster-r7]

Thanks!