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: I6a4954110ff24c724376bf8ed3e42d71eeeb4819
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 4e9ccf5..08e0feb 100644
--- a/meson.build
+++ b/meson.build
@@ -6,7 +6,7 @@
         'werror=true',
         'cpp_std=c++20'
     ],
-    meson_version: '>= 0.57.0',
+    meson_version: '>= 0.58.0',
     license: 'Apache-2.0',
     version: '0.1',
 )
@@ -288,7 +288,7 @@
 
 systemd = dependency('systemd')
 systemd_system_unit_dir = systemd.get_variable(
-    pkgconfig : 'systemdsystemunitdir')
+    'systemdsystemunitdir')
 subdir('service_files')
 subdir('target_files')