migration note on subqueries

migration note on subqueries

Under 2.0, calls to `in_` and `not_in` no longer accept an explicit `.subquery()`.

Passing a `.subquery()` will cause typing issues from MyPy AND will raise runtime warnings.

There was no note of this in the migration guide.  This may be an effect of another change that is disclosed in the migration guide.  If so, I suggest nesting this text (or improved text describing this) under that section for ease in discovery and migration.

Added a note to the 2.0 migration guide.

<!-- go over following points. check them with an `x` if they do apply, (they turn into clickable checkboxes once the PR is submitted, so no need to do everything at once)

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This pull request is:

- [x] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #11107
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/11107
Pull-request-sha: f8669da225bc7687208d2389c6ae3bf3c63f6aaf

Change-Id: Id33779cb126a700d9adebe5f08f9d6c085589db4
(cherry picked from commit c302b2ca46754ea49c5b24fdd8b6375aafb1df96)