DEPR: raise ValueError if invalid period freq pass to asfreq

[natmokval]

xref #52064, #9586

in example:

index = pd.PeriodIndex(["2020-01-01", "2021-01-01"], freq="M")
df = pd.DataFrame({"a": pd.Series([0, 1], index=index)})
df.asfreq(freq=MonthBegin(2))

we silently convert " 2MS" to " 2M", which isn't correct.

Raised ValueError if invalid period freq pass to asfreq()

[MarcoGorelli]

nice, well done noticing this

OK to not have whatsnew as the "silent conversion" isn't present in the latest release (only on main)