Use node 20 not node 21

Author: garronej

action.yml

@@ -80,5 +80,5 @@ outputs:
   is_pre_release:
     description: 'Output of is_package_json_version_upgraded, true|false'
 runs:
-  using: 'node21'
+  using: 'node20'
   main: 'dist/index.js'

src/bin/generateActionYml.ts

@@ -27,7 +27,7 @@ fs.writeFileSync(
             `    description: '${getOutputDescription(outputName).replace(/'/g,"''")}'`
         ].join("\n")),
         `runs:`,
-        `  using: 'node21'`,
+        `  using: 'node20'`,
         `  main: 'dist/index.js'`
     ].join("\n"), "utf8")
 );