docs: Remove migration docs of outdated versions (#8403)

README.md

@@ -92,7 +92,6 @@ A big *thank you* 🙏 to our [sponsors](#sponsors) and [backers](#backers) who
   - [Using automatically generated operations](#using-automatically-generated-operations)
   - [Customizing your GraphQL Schema](#customizing-your-graphql-schema)
   - [Learning more](#learning-more)
-- [Upgrading to Parse Server 3.0](#upgrading-to-parse-server-30)
 - [Contributing](#contributing)
 - [Contributors](#contributors)
 - [Sponsors](#sponsors)
@@ -1154,12 +1153,6 @@ You also have a very powerful tool inside your GraphQL Playground. Please look a
 
 Additionally, the [GraphQL Learn Section](https://graphql.org/learn/) is a very good source to learn more about the power of the GraphQL language.
 
-# Upgrading to Parse Server 3.0
-
-Starting Parse Server 3.0, Parse Server uses the Parse JavaScript SDK 2.0. In short, the Parse JavaScript SDK 2.0 removes the backbone style callbacks as well as the `Parse.Promise` object in favor of native promises. All the Cloud Code interfaces also have been updated to reflect those changes, and all backbone style response objects are removed and replaced by promise style resolution.
-
-We have written up a [migration guide](3.0.0.md) to help you transition to the next major release.
-
 # Contributing
 
 Please see the [Contributing Guide](CONTRIBUTING.md).