This reverts commit 756b424c.

The freedesktop SDK, which is used by gnome-build-meta, only has Meson
0.63. Bumping GLib’s Meson dependency to 0.64 means that, at the moment,
GLib is not buildable in gnome-build-meta and hence can’t be tested in
nightly pipelines against other projects, etc.

That’s bad for testing GLib.

It’s arguably bad that we’re restricted to using an older version of
Meson than shipped by Debian Testing, but that’s a separate discussion
to be had.

Revert the Meson 0.64 dependency until the freedesktop SDK ships Meson ≥
0.64. This also means reverting the simplifications to use of
`gnome.mkenum_simple()`.

See !3077 (comment 1601064)