gmp: fixup license of v6 (#199052)

* gmp: fixup license of v6

As per https://gmplib.org/:
```
Since version 6, GMP is distributed under the dual licenses, GNU LGPL v3 and GNU GPL v2. These licenses make the library free to use, share, and improve, and allow you to pass on the result. The GNU licenses give freedoms, but also set firm restrictions on the use with non-free programs.
```

The license changed in GMP v6.

* Fixup license versions

pkgs/development/libraries/gmp/6.x.nix

@@ -61,7 +61,10 @@ let self = stdenv.mkDerivation rec {
   meta = with lib; {
     homepage = "https://gmplib.org/";
     description = "GNU multiple precision arithmetic library";
-    license = licenses.gpl3Plus;
+    license = with licenses; [
+      lgpl3Only
+      gpl2Only
+    ];
 
     longDescription =
       '' GMP is a free library for arbitrary precision arithmetic, operating