Per supported platforms doc[1], the various min glib on relevant distros is:

  RHEL-7: 2.50.3
  Debian (Stretch): 2.50.3
  Debian (Jessie): 2.42.1
  OpenBSD (Ports): 2.54.3
  FreeBSD (Ports): 2.50.3
  OpenSUSE Leap 15: 2.54.3
  SLE12-SP2: 2.48.2
  Ubuntu (Xenial): 2.48.0
  macOS (Homebrew): 2.56.0

This suggests that a minimum glib of 2.42 is a reasonable target.

The GLibC compile farm, however, uses Ubuntu 14.04 (Trusty) which only
has glib 2.40.0, and this is needed for testing during merge. Thus an
exception is made to the documented platform support policy to allow for
all three current LTS releases to be supported.

Docker jobs that not longer satisfy this new min version are removed.

[1] https://qemu.weilnetz.de/doc/qemu-doc.html#Supported-build-platformsReviewed-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>