DEPR: deprecate the alias 'A' in favour of 'Y' for year end frequency

Context: we renamed `'M'` to `'ME' `for offsets (see #52064) and now we are deprecating `'T'/'L'` in favour of `'min'/'ms'` (see #54061). 

The next step is rename `'A'` to `'AE'` for offsets. We can either:

- rename 'A' to 'AE' and 'Y' to 'YE'
- deprecate `A' in favour of 'Y' and then rename 'Y' to 'YE' for offsets

Should we standartirize `'Y'` for Year?