DEPR: deprecate string H, BH, CBH in offsets frequencies, resolution abbreviations, _attrname_to_abbrevs
xref #54061

- deprecated for Timedelta units 'H' in favour of 'h'
- deprecated for time series frequencies aliases 'H', 'BH', 'CBH' in favour of 'h', 'bh', 'cbh'
- deprecated resolution 'H' for Timedelta.resolution_string in favour of 'h'
I added tests for Hour and CustomBusinessHour and corrected the definition of `to_offset`. @MarcoGorelli, could you please take a look at the update? looks like we're close, just got some conflicts from the A -> Y one (but I think you've mastered resolving conflicts now, so should be fine 😄 ) > looks like we're close, just got some conflicts from the A -> Y one (but I think you've mastered resolving conflicts now, so should be fine 😄 )

thanks, I resolved conflicts, ci is green.