feat(gatsby): release plugin option validation (gatsbyjs#27437)

* chore(gatsby): add @gatsbyVersion pragma to pluginOptionsSchema Node API

This will make Gatsby prompt users on old versions of gatsby that don't support the API to upgrade to the new version.

* 2.25.0!

* Remove GATSBY_EXPERIMENTAL_PLUGIN_OPTION_VALIDATION feature flag

* Fix incorrect option in gatsby-admin;

* Fix tests

* Remove obsolete snapshots

* Trigger Build

* Remove flag from remark-autolink-headers

* Fix gatsby-plugin-mdx default

src/gatsby-node.js

@@ -153,14 +153,12 @@ const createFSMachine = (
   return interpret(fsMachine).start()
 }
 
-if (process.env.GATSBY_EXPERIMENTAL_PLUGIN_OPTION_VALIDATION) {
-  exports.pluginOptionsSchema = ({ Joi }) =>
-    Joi.object({
-      name: Joi.string(),
-      path: Joi.string(),
-      ignore: Joi.array().items(Joi.string()),
-    })
-}
+exports.pluginOptionsSchema = ({ Joi }) =>
+  Joi.object({
+    name: Joi.string(),
+    path: Joi.string(),
+    ignore: Joi.array().items(Joi.string()),
+  })
 
 exports.sourceNodes = (api, pluginOptions) => {
   // Validate that the path exists.