BUG/TST/DEPR: Ensure dtype="category" always implies ordered=False & add tests
- [ ] closes #61074  
- [x] [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
- [x] 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.

Issue: Previously, if a Series had ordered=True, using astype("category") would incorrectly preserve the ordering. This was inconsistent and against expectations.

Fix:
1. Now, dtype="category" always implies ordered=False.
2. If ordered=True before conversion, a deprecation warning is raised to inform users.
3. Tests have been updated to verify this new behavior.

Going to close this PR pending discussion on the original issue. We can reopen once there is more consensus on moving forward here