update version number for final 21.0-Scandium release

ALL_VERSIONS

 Version number			Date of release		Notes
 ==============			===============		=====
+21.0 (Scandium)                 2020, June 11
 20.0 (Calcium)                  2019, December 4
 19.1 (Potassium)                2019, September 18      OCaml 4.08 compatibility
 19.0 (Potassium)                2019, June 21

VERSION

-21.0-beta
+21.0

opam/opam

@@ -16,7 +16,7 @@ Thanks to this approach, Frama-C provides sophisticated tools, including:
 These plug-ins communicate between each other via the Frama-C API
 and via ACSL (ANSI/ISO C Specification Language) properties.
 """
-version: "21.0-beta"
+version: "21.0"
 maintainer: "francois.bobot@cea.fr"
 authors: [
   "Michele Alberti"