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: I27f71195ffb92a3e3224022edca33d8cc249bacd
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index c4ff355..f71acb2 100644
--- a/meson.build
+++ b/meson.build
@@ -1,7 +1,7 @@
 project(
     'eStoraged',
     'cpp',
-    version: '1.0', meson_version: '>=0.57.0',
+    version: '1.0', meson_version: '>=0.58.0',
     default_options: [
       'warning_level=3',
       'werror=true',