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

[natmokval]

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?

[natmokval]

Seems like there are no objections. I'll start to work on that.