meson: update glib minimum version to 2.56

In order to support the symbol g_enum_to_string in various
project using GStreamer ( gst-validate etc.), the glib minimum
version should be 2.56.0.

Remove compat code as glib requirement
is now > 2.56

Version used by Ubuntu 18.04 LTS

Part-of: <gstreamer/gst-devtools!224>
36 jobs for !224 with bump_glib_version_2_56 in 44 minutes and 37 seconds (queued for 8 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #5296936
alpine amd64 manifest builder docker

00:00:21

passed #5296934
android docker

00:02:06

passed #5296933
cerbero fedora amd64 docker

00:02:39

passed #5296931
fedora amd64 docker

00:03:13

passed #5296937
gst-indent amd64 docker

00:00:25

 
  Preparation
passed #5296941
gst indent

00:00:13

passed #5296939
manifest

00:00:32

 
  Build
passed #5296958
gstreamer
build cerbero cross-android universal

00:11:41

passed #5296965
gst-ios-13.2
build cerbero cross-ios universal

00:18:13

passed #5296960
gstreamer
build cerbero cross win32

00:07:14

passed #5296961
gstreamer
build cerbero cross win64

00:06:47

passed #5296955
gstreamer
build cerbero fedora x86_64

00:05:36

passed #5296963
gst-macos-10.15
build cerbero macos x86_64

00:11:54

passed #5296967
1809 gstreamer-windows windows docker
build cerbero msvc x86_64

00:12:44

passed #5296948
build clang fedora x86_64

00:06:10

manual #5296953
1809 windows docker allowed to fail manual
build msys2
passed #5296942
build nodebug fedora x86_64

00:04:40

passed #5296944
build static fedora x86_64

00:04:24

passed #5296946
build static nodebug fedora x86_64

00:04:37

passed #5296949
1809 windows docker
build vs2017 amd64

00:11:03

passed #5296951
1809 windows docker
build vs2017 x86

00:10:44

 
  Test
passed #5301131
gstreamer
check fedora

00:09:07

passed #5296972
gstreamer
integration testsuites fedora 1/4

00:13:48

passed #5296974
gstreamer
integration testsuites fedora 2/4

00:13:02

passed #5296977
gstreamer
integration testsuites fedora 3/4

00:12:16

passed #5296978
gstreamer
integration testsuites fedora 4/4

00:15:05

passed #5296983
gstreamer
valgrind bad

00:04:27

passed #5296980
gstreamer
valgrind base

00:08:25

passed #5296979
gstreamer
valgrind core

00:07:30

passed #5296984
gstreamer
valgrind ges

00:06:13

passed #5296981
gstreamer
valgrind good

00:09:18

passed #5296982
gstreamer
valgrind ugly

00:03:05

failed #5296970
gstreamer
check fedora

00:07:45

 
  Integrate
passed #5296985
cross-android universal examples

00:07:06

passed #5296987
gst-ios-13.2
cross-ios universal examples

00:01:28

passed #5296986
documentation

00:12:11