Removed versionadded:: 2.1 directives

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | -

This PR removes all `versionadded:: 2.1` directives from the 2.3 docs, as stated in the Documentation rules.

 > I doubt if it's usefull to remove them, I think this needs to get done after each time you merge 2.2 into master...