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: I2dff8446b6f0681dac70e5dd1c1e73e325db957e
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 31586cf..b23d656 100644
--- a/meson.build
+++ b/meson.build
@@ -2,7 +2,7 @@
   'kcsbridge',
   'cpp',
   version: '0.1',
-  meson_version: '>=0.57.0',
+  meson_version: '>=0.58.0',
   default_options: [
     'warning_level=3',
     'cpp_std=c++20',
@@ -19,5 +19,5 @@
     output: 'phosphor-ipmi-kcs@.service',
     configuration: conf_data,
     install: true,
-    install_dir: systemd.get_variable(pkgconfig: 'systemdsystemunitdir'))
+    install_dir: systemd.get_variable('systemdsystemunitdir'))
 endif