REG: dt64 shift with integer fill_value
- [x] closes #31971
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


> Do we have a policy on introducing warnings in bugfix releases?

According to https://pandas.pydata.org/docs/development/policies.html 

> We will not introduce new deprecations in patch releases.

But perhaps it's OK here since this is fixing a bug (failure to deprecate something in 1.0). I don't have a strong preference. @jbrockmendel can you update doc-strings, otherwise lgtm. updated+green; i think ive addressed the relevant comments thanks, let's see if this backports @meeseeksdev backport 1.0.x Something went wrong ... Please have  a look at my logs. @TomAugspurger  did this get backported successfully or do i still need to do it manually? https://github.com/pandas-dev/pandas/pull/32647 backported it. I think the thing about the error is from the auto-backport and Jeff's manual request kicking in around the same time, so the second one errored since the backport was already created (maybe). right i am sure not if things backport so i usuually kick the bot which i think is safe (it won’t do it twice)

but found that if it needs a manual backport it shows immediately which is good