misc: Updated version to 20.0.0.0

Change-Id: I5ba4a6e728b2daccea898685d3c27b0f7c7a02cd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28707
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/version.cc

@@ -26,4 +26,4 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-const char *gem5Version = "[develop]";
+const char *gem5Version = "20.0.0.0";