Test 2021.2 (#5329)

.yamato/com.unity.ml-agents-test.yml

     - version: 2020.2
       testProject: DevProject
       enableNoDefaultPackages: !!bool true
-    - version: 2021.1
+    - version: 2021.2
       testProject: DevProject
       enableNoDefaultPackages: !!bool true
 

.yamato/test_versions.metafile

     extra_test: gym
   - version: 2020.3
     extra_test: sensor
-  - version: 2021.1
+  - version: 2021.2
     extra_test: llapi