DEPR: rename `startingMonth` to `starting_month` (argument in BQuarterBegin)
- [x] closes #57794
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

You need to deprecate `startingMonth` first Thanks @Aloqeely ! Could you explain to me how I can deprecate `startingMonth`? Basically you should still allow `startingMonth`, but if it is used you should warn the user that it will be removed soon
I think a check in `__init__` will do the job but I'm not sure, cc @jbrockmendel  Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.