[RF] Add deprecation header for RooMinuit to be backwards compatible
[RF] Add deprecation header for RooMinuit to be backwards compatible

If you include the `RooMinuit.h` header now, you'll get a deprecation
warning and the information that you can still find the class in
`RooFitLegacy/RooMinuit.h`.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root