DEPR: the method is_anchored() for offsets
- [x] closes #55388

deprecated the method `is_anchored()` for offsets classes: `Tick, Week, QuarterOffset, FY5253Mixin, BaseOffset`
in this PR `is_anchored()` is deprecated for offsets. @MarcoGorelli, could you please take a look at my PR? nice! @jbrockmendel I think you'd suggested this, fancy taking a look? I found some inconsistency in the behavior of the classes `BusinessHour` and `CustomBusinessHour`.
We always return `False` for `pd.offsets.Hour().is_anchored()` 
But unexpectedly I get `True` for 
```
pd.offsets.BusinessHour().is_anchored()
pd.offsets.CustomBusinessHour().is_anchored()
```
I am not sure, is it a bug?

And messages in the `FutureWarning` are different:
```
Hour.is_anchored is deprecated and will be removed in a future version, please use False instead.
BusinessHour.is_anchored is deprecated and will be removed in a future version, please use 'obj.n == 1' instead.
```

The reason is they are non-Tick subclasses.

Should we fix it before we deprecate is_anchored? What do you think, @jbrockmendel? just for reference, here's the (only) use-case I found of `is_anchored` in the wild:

https://github.com/pastas/pastas/blob/dbe3275e012143605e3a162bfbb8ff0983d966e6/pastas/timeseries_utils.py#L160-L185

It wouldn't work for converting `'2W-Tue'` to `'14D'` though, so I'm not sure `is_anchored` is what they really wanted in the first place

I doubt anyone's using it for business hour / custom business hour - I think it's fine to just deprecate /preview Website preview of this PR available at: https://pandas.pydata.org/preview/56594/ thanks @MarcoGorelli for reviewing this PR