meson: require 0.53.2 and drop some workarounds for old meson

Ubuntu Bionic 18.04 has 0.45, so it was below the previously required
minimum version already. Focal 20.04 has 0.53.2. Let's require that
and use various features that are available.

tmpfiles.d/meson.build

@@ -37,8 +37,7 @@ foreach pair : in_files
                 # do nothing
         elif pair[1] == '' or conf.get(pair[1]) == 1
                 custom_target(
-                        # XXX: workaround for old meson. Drop when upgrading.
-                        'tmpfiles+' + pair[0],
+                        pair[0],
                         input : pair[0] + '.in',
                         output: pair[0],
                         command : [meson_render_jinja2, config_h, '@INPUT@'],