CLN: named parameters for GroupBy.(mean|median|var|std)
Drops *args & **kwargs, replace with named parameters for groupby methods mean, median, var & std. Similar to #31473.

This PR has the side effect that the raised error when a parameter is not allowed, is now ``TypeError`` instead of ``UnsupportedFunctionCall``, so technically a API change...
Maybe a decorator similar to ``deprecate_kwargs`` could be added. That way we'd get the correct signature, and ``UnsupportedFunctionCall`` would be raised in the decorator. > looks good to me. I'd be ok with 1.1 but maybe worth a 2.0 target as well

we don’t have the bandwidth or infrastructure to handle this type of versioning
if it’s not targeted then would rather close

this is fine for 1.1 Ok, so I don't do a decorator solution? Ok, I've added doc strings.

Also, I've found out that ``Groupby.median`` actually didn't accept ``min_count`` in master, but raised an ``AssertionError``. So I removed the parameter from that method signature.
 thanks @topper-123  > Ok, I've added doc strings.
> 
> Also, I've found out that `Groupby.median` actually didn't accept `min_count` in master, but raised an `AssertionError`. So I removed the parameter from that method signature.

happy to have an issue / PR about this. I did fix that is this PR, so no issue necesary.

So in short, in v1.0.0/master we'd have that ``grp.mean(min_count=1)`` raised a ``AssertionError``, while now it raised a ``TypeError``, which is the same error code as we'd get for ``df.mean(min_count=1)``, so is the correct error type, IMO.
