Upgrade the minimum required GTK version for JavaFX to GTK 3.20 to enable modern features and better Linux desktop integration.

JavaFX currently depends on GTK 3.8, released in March 2013 (https://mail.gnome.org/archives/gtk-devel-list/2013-March/msg00108.html). This version is outdated and predates many useful GTK API improvements.

GTK 3.20 was released on March 21, 2016 (https://mail.gnome.org/archives/gtk-list/2016-March/msg00026.html).

Updating the GTK minimum requirement to 3.20 would enable JavaFX to support new features, including the improvements proposed in https://github.com/openjdk/jfx/pull/1605.

Major Linux distributions already provide GTK 3.20 or newer:
- Ubuntu LTS 18.04+ (ships GTK 3.22+)
- Debian 9+ (ships GTK 3.22+)
- Fedora 24+ (ships GTK 3.20+)
- Oracle Linux and Red Hat Enterprise Linux 8+ (ships GTK 3.22+)