meson: Move to c++20

Change-Id: I823469db19cfc8ae785d6f884069241b239fd938
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/meson.build b/meson.build
index 113814f..9245936 100644
--- a/meson.build
+++ b/meson.build
@@ -2,10 +2,10 @@
   'phosphor-watchdog',
   'cpp',
   version: '0.1',
-  meson_version: '>=0.55.0',
+  meson_version: '>=0.57.0',
   default_options: [
     'warning_level=3',
-    'cpp_std=c++17',
+    'cpp_std=c++20',
   ])
 
 watchdog_headers = include_directories('.')