DEPR: deprecate obj argument in GroupBy.get_group
- [x] closes #53545
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

`obj` argument was deprecated in `GroupBy.get_group` and removed from the parameters in `GroupBy.get_group` docs.
@rhshadrach,  could you please take a look at this pr? Thanks for the PR! This is indeed how the code should look after the deprecation is enforced. However we need to first introduce the deprecation so as to not break any user code but rather warn them about the change. If you're unfamiliar with this, I would suggest looking at merged PRs with the "Deprecate" label to get a better sense as to what this entails. Thank you, @rhshadrach for the comment. Sorry, my fault, I corrected my pr. Could you please take a look at update. Thank you @rhshadrach for reviewing this. Sorry for the delay with adding the test.