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: I597f925bed69522d527ba88bc8f0c6d2f0e165c0
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 75b57bf..aee5da1 100644
--- a/meson.build
+++ b/meson.build
@@ -2,7 +2,7 @@
 
 project('phosphor-debug-collector',
         'cpp',
-        meson_version: '>= 0.57.0',
+        meson_version: '>= 0.58.0',
         default_options: [
                           'cpp_std=c++20',
                           'warning_level=3',
@@ -260,9 +260,9 @@
 
 unit_subs = configuration_data()
 unit_subs.set('bindir', join_paths(get_option('prefix'), get_option('bindir')))
-systemd_system_unit_dir = dependency('systemd').get_pkgconfig_variable(
+systemd_system_unit_dir = dependency('systemd').get_variable(
     'systemdsystemunitdir',
-    define_variable: ['prefix', get_option('prefix')])
+    pkgconfig_define: ['prefix', get_option('prefix')])
 foreach u : unit_files
     configure_file(
         configuration: unit_subs,