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: I2c107a339cfda6cc9f1b2fdcd2123f603bce9216
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 3f08d17..323118b 100644
--- a/meson.build
+++ b/meson.build
@@ -7,7 +7,7 @@
         'cpp_std=c++20',
         'werror=true',
     ],
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
 )
 
 install_data(
@@ -41,6 +41,6 @@
     output: 'start-ipkvm.service',
     copy: true,
     install_dir:
-        dependency('systemd').get_pkgconfig_variable(
+        dependency('systemd').get_variable(
             'systemdsystemunitdir')
 )