ENH: unify warning message in to_offset()
xref  #56346

unified warning message for deprecated frequencies in to_offset in order to make test parametrization more clear
@MarcoGorelli, could you please take a look at this PR?
We have two slightly different warnings messages for deprecated frequencies (they differ only on "` and will be removed in a future version"`). I think it makes sense to make them look the same.

Then we can combine some tests in #56346 together, e.g. in `pandas/tests/indexes/period/test_period_range.py` we can join together `test_uppercase_us_ns_deprecated_from_time_series` and `test_mixcase_us_ns_deprecated_from_time_series`.
