build: Update to c++20, c18, and meson >= 0.57.0

Change-Id: I6b4862f1aa2f8778d63b2b905762437071afd9c6
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/subprojects/nemora-postd/meson.build b/subprojects/nemora-postd/meson.build
index f949663..487b7c0 100644
--- a/subprojects/nemora-postd/meson.build
+++ b/subprojects/nemora-postd/meson.build
@@ -19,7 +19,7 @@
   default_options: [
     'warning_level=3',
     'werror=true',
-    'cpp_std=c++17'
+    'cpp_std=c++20'
   ],
 )