docs/system/keys.rst.inc still refers to removed options -alt-grab and -ctrl-grab

We removed the command line options -ctrl-grab and -alt-grab in QEMU 7.1, but the documentation file docs/system/keys.rst.inc still has a mention of them.

We should update that documentation to reword it appropriately. docs/about/removed-features.rst says the new syntax is -display sdl,grab-mod=lshift-lctrl-lalt or =rctrl. Probably best to just say "The default key mappings shown below use Ctrl-Alt key combinations, but if you are using the SDL UI you can change this using the -display sdl,grab-mod=<modifiers> option." or something similar.