RFR: 8359396: [Linux] Require Gtk3 >= 3.20 for glass-gtk

[Thiago Milczarek Sayao]

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](https://mail.gnome.org/archives/gtk-devel-list/2013-March/msg00108.html) in March 2013. This version is outdated and predates many useful GTK API improvements.

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

Updating the GTK minimum requirement to 3.20 would enable JavaFX to support new features, including the improvements proposed in  #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+)

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Commit messages:
 - Remove return
 - Require Gtk 3.20

Changes: https://git.openjdk.org/jfx/pull/1825/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1825&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359396
  Stats: 61 lines in 3 files changed: 36 ins; 23 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1825.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1825/head:pull/1825

PR: https://git.openjdk.org/jfx/pull/1825