docs: reflect release of v9 in config migration guide (#18412)

Update the eslintrc-to-flat config migration guide with the release of 9.0.0.

docs/src/use/configure/migration-guide.md

@@ -18,7 +18,7 @@ For reference information on these configuration formats, refer to the following
 
 ## Start Using Flat Config Files
 
-Starting with ESLint v9.0.0, the flat config file format will be the default configuration file format. Once ESLint v9.0.0 is released, you can start using the flat config file format without any additional configuration.
+The flat config file format has been the default configuration file format since ESLint v9.0.0. You can start using the flat config file format without any additional configuration.
 
 To use flat config with ESLint v8, place a `eslint.config.js` file in the root of your project **or** set the `ESLINT_USE_FLAT_CONFIG` environment variable to `true`.