meson: switch to 0.57.0

- The intent behind this commit is switch meson version to 0.57.0
  and switch C++ version to 20.

- Also, Because the meson call `source_root` is deprecated
  since 0.56.0, switch to `project_source_root` and is the correct
  replacement for the deprecated `source_root`.

Tested: built ledManager repo successfully and worked.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I5ef0569df0be22470104aea493812efa24516639
diff --git a/meson.build b/meson.build
index 6412a4c..387c469 100644
--- a/meson.build
+++ b/meson.build
@@ -1,11 +1,11 @@
 project(
     'phosphor-ledmanager', 'cpp',
     version : '1.0.0',
-    meson_version: '>=0.54.1',
+    meson_version: '>=0.57.0',
     default_options: [
         'warning_level=3',
         'werror=true',
-        'cpp_std=c++17',
+        'cpp_std=c++20',
         'buildtype=debugoptimized'
     ]
 )
@@ -116,8 +116,8 @@
         'led-gen.hpp',
         command : [
             prog_python,
-            meson.source_root() + '/parse_led.py',
-            '-i', meson.source_root(),
+            meson.project_source_root() + '/parse_led.py',
+            '-i', meson.project_source_root(),
             '-o', meson.current_build_dir(),
         ],
         output : 'led-gen.hpp')