MAINT: pandas offset alias updates

[jklenzing]

Description

Pandas is updating the offset alias strings for pandas 3.0.0. These are implemented in pandas 2.2.0, and usage of old syntax will generate a FutureWarning.

According to SPEC-0000, support for pandas 2.1.0 should be maintained through Aug 2025.

Path forward

• Use if / else notation based on pandas version to ensure forward/backward compatibility
• Drop these and set a minimum pandas version once support is dropped for pandas<2.2.

Additional context

https://pandas.pydata.org/pandas-docs/stable/user_guide/timeseries.html#offset-aliases