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: Ibd32733f23eed0d329756e97a65b12f2631f9a61
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 2c5b23b..d877d7b 100644
--- a/meson.build
+++ b/meson.build
@@ -1,7 +1,7 @@
 project(
     'Telemetry',
     'cpp',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
     default_options: [
         'buildtype=debugoptimized',
         'cpp_std=c++20',
@@ -128,7 +128,7 @@
         'after': service_after
     },
     install: true,
-    install_dir: systemd.get_variable(pkgconfig: 'systemdsystemunitdir'),
+    install_dir: systemd.get_variable('systemdsystemunitdir'),
 )
 
 if get_option('buildtest')