Revert "bump min meson version"

OpenSlide supports Meson down to 0.53, which is the version in Ubuntu
20.04.  OpenSlide disables libdicom's tests when falling back to the
libdicom wrap, circumventing check's requirement for a newer Meson.

This reverts commit ec925f3.

Author: bgilbert

meson.build

@@ -7,8 +7,7 @@ project(
     'warning_level=2',
   ],
   license : 'MIT',
-  # "check" needs 0.54.1
-  meson_version : '>=0.54.1',
+  meson_version : '>=0.50',
   version : '1.0.0'
 )
 if not meson.is_subproject()