FindMatlab: add R2020b => 9.9

Modules/FindMatlab.cmake

@@ -241,6 +241,7 @@ if(NOT MATLAB_ADDITIONAL_VERSIONS)
 endif()
 
 set(MATLAB_VERSIONS_MAPPING
+  "R2020b=9.9"
   "R2020a=9.8"
   "R2019b=9.7"
   "R2019a=9.6"