make deprecated positional arguments keyword-only

[d-v-b]

we have a lot of functions decorated with this decorator, which by default warns that the decorated function will only take keyword-only arguments from version 3.1.0 and onwards. not accept certain arguments as positional from 3.1.0 and onwards.

As we are preparing a 3.1.0 release, we should transform these functions to all keyword-only arguments, and remove the decoration.

[d-v-b]

worth noting that create_array doesn't use keyword-only arguments (the store is positional). Is that something we want to eventually change?

[d-v-b]

similarly, create_group is not all keyword-only, and I don't think we had deprecations for that

[d-v-b]

ok so over in #3093 @dstansby corrected my misinterpretation of the _deprecate_positional_args decorator. For 3.1.0 we don't need to change any function signatures, just remove the use of decorator.