BUG: DatetimeIndex.is_year_start breaks on custom business days frequencies bigger then `1C`
- [x] closes #58664
- [x] Tests added and passed
- [x] Added an entry in the latest `doc/source/whatsnew/v3.0.0.rst

this needs rebasing, as it contains commits from the other PR (and could you move the whatsnew from that one to `DatetimeLike` please?)

(other than that, it looks like this identifies + fixes the issue, nice) > this needs rebasing, as it contains commits from the other PR (and could you move the whatsnew from that one to `DatetimeLike` please?)
> 
thanks, rebased and moved the whatsnew note from `Indexing` to `DatetimeLike`  Thanks @natmokval 