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
35 jobs for !224 with bump_glib_version_2_56 in 4 minutes and 9 seconds (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #5042027
alpine amd64 manifest builder docker

00:00:29

passed #5042026
android docker

00:02:13

passed #5042025
cerbero fedora amd64 docker

00:03:52

passed #5042024
fedora amd64 docker

00:03:16

passed #5042028
gst-indent amd64 docker

00:00:28

 
  Preparation
passed #5042030
gst indent

00:00:16

manual #5042029
manual
manifest
 
  Build
created #5042039
gstreamer
build cerbero cross-android universal
created #5042043
gst-ios-13.2
build cerbero cross-ios universal
created #5042040
gstreamer
build cerbero cross win32
created #5042041
gstreamer
build cerbero cross win64
created #5042038
gstreamer
build cerbero fedora x86_64
created #5042042
gst-macos-10.15
build cerbero macos x86_64
created #5042044
1809 gstreamer-windows windows docker
build cerbero msvc x86_64
created #5042034
build clang fedora x86_64
created #5042037
1809 windows docker allowed to fail manual
build msys2
created #5042031
build nodebug fedora x86_64
created #5042032
build static fedora x86_64
created #5042033
build static nodebug fedora x86_64
created #5042035
1809 windows docker
build vs2017 amd64
created #5042036
1809 windows docker
build vs2017 x86
 
  Test
created #5042045
gstreamer
check fedora
created #5042046
gstreamer
integration testsuites fedora 1/4
created #5042047
gstreamer
integration testsuites fedora 2/4
created #5042048
gstreamer
integration testsuites fedora 3/4
created #5042049
gstreamer
integration testsuites fedora 4/4
created #5042054
gstreamer
valgrind bad
created #5042051
gstreamer
valgrind base
created #5042050
gstreamer
valgrind core
created #5042055
gstreamer
valgrind ges
created #5042052
gstreamer
valgrind good
created #5042053
gstreamer
valgrind ugly
 
  Integrate
created #5042056
cross-android universal examples
created #5042058
gst-ios-13.2
cross-ios universal examples
created #5042057
documentation