meson: remove deprecated get_pkgconfig_variable

Since meson 0.56, the `get_pkgconfig_variable` has been deprecated.  In
meson 0.58 the `get_variable` was enhanced to no longer require the
`pkgconfig` keyword argument.  Ensure meson 0.58 is required and update
the usage of all `get_pkgconfig_variable` and `get_variable` to be the
modern variant.

Change-Id: Ica7f741355ff02a7c4eb84c6c1dfc47e0b42a3f8
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index a58580a..93a4960 100644
--- a/meson.build
+++ b/meson.build
@@ -1,7 +1,7 @@
 project('phosphor-time-manager',
         'cpp',
          version: '0.1',
-         meson_version: '>=0.57.0',
+         meson_version: '>=0.58.0',
          default_options: [ 'warning_level=3',
                             'werror=true',
                             'cpp_std=c++20',
@@ -66,7 +66,7 @@
 
 systemd = dependency ('systemd')
 systemd_system_unit_dir = systemd.get_variable(
-    pkgconfig : 'systemdsystemunitdir',
+    'systemdsystemunitdir',
     pkgconfig_define: ['prefix', get_option('prefix')])
 configure_file(input : 'xyz.openbmc_project.Time.Manager.service',
                output : 'xyz.openbmc_project.Time.Manager.service',