DEPR: make Series.agg aggregate when possible
The doc string for `Series.agg` says: `"A passed user-defined-function will be passed a Series for evaluation."` which isn't true currently. In addition, `Series.agg` currently will not always aggregate, when given a aggregation function. This PR fixes those issues.

It can be noted that both the behavior and that doc string are correct for `DataFrame.agg`, so this PR aligns the behavior of `Series.agg` and `DataFrame.agg`.

I'm not sure if this is too drastic as a bug fix, or we want it as part of a deprecation process, but just putting this up there, will see where it goes.

EDIT: In the new version the old behavior is deprecated rather than just removed. 
I stumbled upon this in https://github.com/pandas-dev/pandas/pull/53208. I think changing this behavior is good but should be deprecated IMO to ensure `f` actually produces a reduced result I've made a new version with deprecation. In the new deprecation version I BTW found out that while `DataFrame.agg` isn't affected by this PR when given a single callable or a dict of callables, it was actually affected when given a list of callables...
 @mroeschke - good here? Thanks @topper-123 