Update cftime frequency strings in line with recent updates in pandas

[spencerkclark]

What is your issue?

Pandas has introduced some deprecations in how frequency strings are specified:

• Deprecating "A", "A-JAN", etc. in favor of "Y", "Y-JAN", etc. (DEPR: 'A' for yearly frequency and YearEnd in favour of 'Y' pandas-dev/pandas#55252)
• Deprecating "AS", "AS-JAN", etc. in favor of "YS", "YS-JAN", etc. (DEPR: ‘AS’, ‘BA’ and ‘BAS’ in favour of ‘YS’, ‘BY’ and ‘BYS’ pandas-dev/pandas#55479)
• Deprecating "Q", "Q-JAN", etc. in favor of "QE", "QE-JAN", etc. (DEPR offsets: rename 'Q' to 'QE' pandas-dev/pandas#55553)
• Deprecating "M" in favor of "ME" (DEPR: deprecate strings T, S, L, U, and N in offsets frequencies, resolution abbreviations, _attrname_to_abbrevs pandas-dev/pandas#54061)
• Deprecating "H" in favor of "h" (DEPR: deprecate string H, BH, CBH in offsets frequencies, resolution abbreviations, _attrname_to_abbrevs pandas-dev/pandas#54939)
• Deprecating "T", "S", "L", and "U" in favor of "min", "s", "ms", and "us" (DEPR: deprecate strings T, S, L, U, and N in offsets frequencies, resolution abbreviations, _attrname_to_abbrevs pandas-dev/pandas#54061).

It would be good to carry these deprecations out for cftime frequency specifications to remain consistent.

[aulemahal]

That's quite heavy a change... This move from pandas seems an unnecessary leap to me!
I'll look into the PR, thanks for heads up!