DEPR: lowercase strings `w`, `d`, `b` and ``c`` denoting frequencies in `Week`, `Day`, `BusinessDay` and `CustomBusinessDay` classes
xref #57986

- deprecated lowercase strings `d`, `b` and `c` denoting frequencies in `Day`, `BusinessDay` and `CustomBusinessDay` in favour of `D`, `B` and `C`
- deprecated lowercase strings `w`, `w-mon`, `w-tue`, etc. denoting frequencies in `Week` in favour of `W`, `W-MON`, `W-TUE`, etc. 

we can  deprecated lowercase strings `w`, `d`, `b` and `c` for `Timedelta` in a separate PR
I deprecated lowercase strings `w`,  `d`, `b` and `c` denoting frequencies in favour of `W`, `D`, `B` and `C`. @MarcoGorelli, could you please take a look at this PR? I think CI failures are unrelated to my changes. @mroeschke thoughts on 2.3 vs 3.0? Thanks @natmokval and @MarcoGorelli 