Added v15 migration document (#7716)

website/src/docs/hotchocolate/v15/migrating/migrate-from-14-to-15.md

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+---
+title: Migrate Hot Chocolate from 14 to 15
+---
+
+This guide will walk you through the manual migration steps to update your Hot Chocolate GraphQL server to version 15.
+
+Start by installing the latest `15.x.x` version of **all** of the `HotChocolate.*` packages referenced by your project.
+
+> This guide is still a work in progress with more updates to follow.
+
+# Breaking changes
+
+Things that have been removed or had a change in behavior that may cause your code not to compile or lead to unexpected behavior at runtime if not addressed.
+
+## Supported target frameworks
+
+Support for .NET Standard 2.0, .NET 6, and .NET 7 has been removed.
+
+## F# support removed
+
+`HotChocolate.Types.FSharp` has been replaced by the community project [FSharp.HotChocolate](https://www.nuget.org/packages/FSharp.HotChocolate).
+
+# Deprecations
+
+Things that will continue to function this release, but we encourage you to move away from.