DEPR: min_periods=None behavior for Rolling.count
- [x] closes #31302
- [x] closes #35579 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Additionally refactors `count` to take the `_apply` path to make this op consistent with the others.
This will close #35579 too.

You could use
```
def test_fixed_forward_indexer_count():
    # GH: 35579
    df = DataFrame({"b": [None, None, None, 7]})
    indexer = FixedForwardWindowIndexer(window_size=2)
    result = df.rolling(window=indexer, min_periods=0).count()
    expected = DataFrame({"b": [0.0, 0.0, 1.0, 1.0]})
    tm.assert_frame_equal(result, expected)
```
as a test. Thanks @phofl. Added that unit test to this PR  thanks @mroeschke 