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: I33cce28df4ed3f5110e582f863809dfdc742af36
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index dfeb8a8..9bdbd3b 100644
--- a/meson.build
+++ b/meson.build
@@ -9,7 +9,7 @@
    'cpp_std=c++20',
   ],
   license: 'Apache-2.0',
-  meson_version: '>=0.57.0',
+  meson_version: '>=0.58.0',
 )
 
 # version info from git
@@ -23,9 +23,9 @@
 
 # install systemd unit template file
 systemd = dependency('systemd')
-systemd_system_unit_dir = systemd.get_pkgconfig_variable(
+systemd_system_unit_dir = systemd.get_variable(
   'systemdsystemunitdir',
-  define_variable: ['prefix', get_option('prefix')],
+  pkgconfig_define: ['prefix', get_option('prefix')],
 )
 configure_file(
   copy: true,