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: I9daae807312503e2ba42b6cae0417c54621db394
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index fd5fbe1..f3127d6 100644
--- a/meson.build
+++ b/meson.build
@@ -5,7 +5,7 @@
     default_options: [
         'cpp_std=c++20',
     ],
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
 )
 
 cpp = meson.get_compiler('cpp')
@@ -53,4 +53,4 @@
   output: 'phosphor-health-monitor.service',
   configuration: conf_data,
   install: true,
-  install_dir: systemd.get_pkgconfig_variable('systemdsystemunitdir'))
+  install_dir: systemd.get_variable('systemdsystemunitdir'))