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: I02322901951608cb3348060f076be10a2a52d022
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/dist/meson.build b/dist/meson.build
index 7ed6046..6b29445 100644
--- a/dist/meson.build
+++ b/dist/meson.build
@@ -1,5 +1,5 @@
 systemd_system_unit_dir = systemd_dep.get_variable(
-    pkgconfig: 'systemdsystemunitdir'
+    'systemdsystemunitdir'
 )
 busconfig_dir = get_option('datadir') / 'dbus-1' / 'system.d'
 cert_manager_dir = get_option('datadir') / 'phosphor-certificate-manager'
diff --git a/meson.build b/meson.build
index e2a337f..8ab2f77 100644
--- a/meson.build
+++ b/meson.build
@@ -7,7 +7,7 @@
         'warning_level=3',
         'werror=true',
     ],
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
 )
 
 cpp = meson.get_compiler('cpp')