versions.json: set version to 2.12.0

Set version to 2.12.0 to prepare for stable release.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>

Author: kv2019i

versions.json

@@ -1,7 +1,7 @@
 {
   "SOF": {
     "MAJOR": "2",
-    "MINOR": "11",
-    "MICRO": "99"
+    "MINOR": "12",
+    "MICRO": "0"
   }
 }