gnome3: only maintain single GNOME 3 package set (#29397)

* gnome3: only maintain single GNOME 3 package set

GNOME 3 was split into 3.10 and 3.12 in #2694. Unfortunately, we barely have the resources
to update a single version of GNOME. Maintaining multiple versions just does not make sense.
Additionally, it makes viewing history using most Git tools bothersome.

This commit renames `pkgs/desktops/gnome-3/3.24` to `pkgs/desktops/gnome-3`, removes
the config variable for choosing packageset (`environment.gnome3.packageSet`), updates
the hint in maintainer script, and removes the `gnome3_24` derivation from `all-packages.nix`.

Closes: #29329

* maintainers/scripts/gnome: Use fixed GNOME 3 directory

Since we now allow only a single GNOME 3 package set, specifying
the working directory is not necessary.

This commit sets the directory to `pkgs/desktops/gnome-3`.