ui: increase min required GTK version to 3.22.0

ui: increase min required GTK version to 3.22.0

Based on a mail on the qemu-devel mailing list at
https://lists.nongnu.org/archive/html/qemu-devel/2020-05/msg02909.html
and some internet research the GTK3 versions on supported
platforms are:

    RHEL-7.4: 3.22.10
    RHEL-7.5: 3.22.26
    Debian (Stretch): 3.22.11
    Debian (Buster): 3.24.5
    OpenBSD (Ports): 3.22.30
    FreeBSD (Ports): 3.22.29
    OpenSUSE Leap 15: 3.22.30
    SLE12-SP2: Unknown
    SLE15: 3.22.30
    Ubuntu (Bionic): 3.22.30
    Ubuntu (Focal): 3.24.18
    macOS (Homebrew): 3.22.30

This justifies increasing the minimum required GTK version in
QEMU to 3.22.0.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-id: 20200516072014.7766-11-vr_qemu@t-online.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>