meson: Update to c++20

Change-Id: Ia527700f3bbc718eff1f298864fd6aec08cb2021
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/meson.build b/meson.build
index dfb9658..9efa9f8 100644
--- a/meson.build
+++ b/meson.build
@@ -4,7 +4,7 @@
   version: '0.1', meson_version: '>=0.57.0',
   default_options: [
     'warning_level=3',
-    'cpp_std=c++17',
+    'cpp_std=c++20',
     'tests=' + (meson.is_subproject() ? 'disabled' : 'auto'),
     'examples=' + (meson.is_subproject() ? 'false' : 'true'),
   ])