Use SPDX license identifier in opam file. (#11861)

haxe.opam

@@ -11,7 +11,7 @@ maintainer: ["Haxe Foundation <[email protected]>" "Andy Li <[email protected]>
 authors: "Haxe Foundation <[email protected]>"
 homepage: "https://haxe.org/"
 bug-reports: "https://github.com/HaxeFoundation/haxe/issues"
-license: ["GPL2+" "MIT"]
+license: ["GPL-2.0-or-later" "MIT"]
 dev-repo: "git+https://github.com/HaxeFoundation/haxe.git"
 build: [
   [make]