elements/components/meson.bst: revert update to 0.51.0

xx.0 releases of meson usually contains many regressons and it's
better to always wait for xx.1 bugfix release.

Also meson doesn't create stable branches for xx.0 releases and
tracking non-existent branch breaks our check-update script.

I removed tracking master branch to avoid future confusions.
12 jobs for EmersonBernier/meson_revert in 292 minutes and 13 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Bootstrap
manual #243724934
cache_x86_64 x86_64 allowed to fail manual
bootstrap_powerpc64le
 
  Flatpak
passed #243724937
aarch64
app_aarch64

04:44:05

passed #243724939
armhf
app_arm

01:30:38

passed #243724936
cache_i686 x86_64
app_i686

03:27:32

manual #243724942
ppc64le allowed to fail manual
app_powerpc64le
passed #243741434
x86_64 cache_x86_64
app_x86_64

03:11:20

failed #243724935
cache_x86_64 x86_64
app_x86_64

00:07:14

 
  Vm
manual #243724949
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #243724950
armhf
minimal_systemd_vm_arm

00:02:58

passed #243724947
cache_i686 x86_64
minimal_systemd_vm_i686

00:07:40

manual #243724952
ppc64le allowed to fail manual
minimal_systemd_vm_powerpc64le
passed #243724945
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:08:06