C++23: `static operator()`

[bernhardmgruber]

Since C++23, call operators can be declared static, which may lead to better generated code. Also, a lambda's operator() will also be declared static if it does not capture anything.

[stale]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.