CI: Add --ignore-deprecated to validate_docstrings.py
The script `scripts/validate_docstrings.py` returns the list of errors in the docstrings. We are using it to get the list of things to fix, and to check in the CI that no more mistakes are added, after the errors are fixed. See for example #23630.

As we don't really want to spend time fixing the docstrings that will be removed soon, it would be useful to add an argument to the script `--ignore-deprecated` that skips the functions/methods that are marked as deprecated (see `Docstring.deprecated`).

Things to note:
- When deprecated docstrings are skipped, they should not be displayed in the output of the script, and also not counted in the exit status of the script (so, the script should finish with exit status 0 if `--ignore-deprecated` is set and only deprecated functions are wrong).
- We should add at least one test to make sure the new argument works as expected.