Filter pre_post_type with sanitize_key()

[duck_]

Currently the post_type field is sanitized with sanitize_user(). It would make more sense to use sanitize_key() for this because

1. register_post_type() uses sanitize_key()
2. It's a lighter function

See #14910.

[duck_]

In [20234]:

Pass post_type through sanitize_key() instead of sanitize_user(). Fixes #20202.

For consistency with the registration API and because it does less work.