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: Iea38cd41ecdf6425bb335af39691dfac60cad7db
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index a38726d..0b49b6a 100644
--- a/meson.build
+++ b/meson.build
@@ -2,7 +2,7 @@
   'phosphor-networkd',
   'cpp',
   version: '0.1',
-  meson_version: '>=0.57.0',
+  meson_version: '>=0.58.0',
   default_options: [
     'warning_level=3',
     'cpp_std=c++20',
@@ -76,7 +76,7 @@
   configuration: conf_data,
   install: true,
   install_dir: dependency('systemd').get_variable(
-    pkgconfig: 'systemdutildir') / 'network')
+    'systemdutildir') / 'network')
 
 configure_file(
   input: 'xyz.openbmc_project.Network.service.in',
@@ -87,7 +87,7 @@
   },
   install: true,
   install_dir: dependency('systemd').get_variable(
-    pkgconfig: 'systemdsystemunitdir'))
+    'systemdsystemunitdir'))
 
 configure_file(
   input: 'xyz.openbmc_project.Network.conf.in',
diff --git a/src/ibm/hypervisor-network-mgr-src/meson.build b/src/ibm/hypervisor-network-mgr-src/meson.build
index 8f929ff..cccd884 100644
--- a/src/ibm/hypervisor-network-mgr-src/meson.build
+++ b/src/ibm/hypervisor-network-mgr-src/meson.build
@@ -9,7 +9,7 @@
   },
   install: true,
   install_dir: dependency('systemd').get_variable(
-    pkgconfig: 'systemdsystemunitdir'))
+    'systemdsystemunitdir'))
 
 hyp_src_includes = include_directories('.')