REGR: preserve freq in DTI/TDI outer join
- [x] closes #32157
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

also just a nice cleanup, hoping to get this into ExtensionIndex before long
i marked for 1.1, but is this a regression? rebase as well > i marked for 1.1, but is this a regression?

The behavior changed between 0.25.3 and 1.0, but it isnt obvious whether to call it a regression or not because it didnt obviously break anything. (see #32157) > > i marked for 1.1, but is this a regression?
> 
> The behavior changed between 0.25.3 and 1.0, but it isnt obvious whether to call it a regression or not because it didnt obviously break anything. (see #32157)

ok it was marked originally as a regression
however i bet this will non trivial to backport (tests)

so up to u removed REGR tag hmm i think this needs a whatsnew note (to document that this is changing), otherwise lgtm. thanks @jbrockmendel  Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.0.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 27f0000804072e8d65e6c7939ec09069e1b9790e
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #32166: REGR: preserve freq in DTI/TDI outer join'
```

4. Push to a named branch :

```
git push YOURFORK 1.0.x:auto-backport-of-pr-32166-on-1.0.x
```

5. Create a PR against branch 1.0.x, I would have named this PR:

> "Backport PR #32166 on branch 1.0.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 @jbrockmendel as expected this needs a manual backport :-<