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: Ibf09a7c616fbde226d962d16b12f4707a5bba2a9
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index c204fb3..4e0f073 100644
--- a/meson.build
+++ b/meson.build
@@ -6,11 +6,11 @@
           'cpp_std=c++20',
           'b_lto=true',
         ],
-        meson_version: '>=0.57.0',
+        meson_version: '>=0.58.0',
        )
 
 systemd = dependency('systemd')
-servicedir = systemd.get_pkgconfig_variable('systemdsystemunitdir')
+servicedir = systemd.get_variable('systemdsystemunitdir')
 subdir('services')
 
 # Common configurations for src and test