Revise README to remove migration update section

Removed outdated migration notice and updated README structure.

Author: ChrisRackauckas

README.md

@@ -4,10 +4,10 @@
 [![Build Status](https://github.com/SciML/DiffEqBase.jl/workflows/Tests/badge.svg)](https://github.com/SciML/DiffEqBase.jl/actions?query=workflow%3ATests)
 [![Build status](https://badge.buildkite.com/99cbd352c336779c3117e4da61255a1ed8e7e7c084c3c2516c.svg)](https://buildkite.com/julialang/diffeqbase-dot-jl)
 
-## Update
-
-This package was moved to be a sublibrary in [OrdinaryDiffEq.jl](https://github.com/SciML/OrdinaryDiffEq.jl). The migration is a maintanance change but should have no functional
-change to the library.
+> [!IMPORTANT] 
+>
+> This package was moved to be a sublibrary in [OrdinaryDiffEq.jl](https://github.com/SciML/OrdinaryDiffEq.jl). The migration is a maintanance change but should have no functional
+> change to the library.
 
 ## Previous Stuff