remove 2018.4 from tests, make sure to user 2020.3

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

 test_editors:
-  - version: 2018.4
-    # 2018.4 doesn't support code-coverage
-    coverageOptions:
-    minCoveragePct: 0
-    # We want some scene tests to run in the DevProject, but packages there only support 2019+
-    testProject: Project
-    testProject: DevProject
+    testProject: Project
   - version: 2020.1
     coverageOptions: --enable-code-coverage --code-coverage-options 'generateHtmlReport;assemblyFilters:+Unity.ML-Agents'
     minCoveragePct: 72

.yamato/standalone-build-test.yml

 test_editors:
-  - version: 2018.4
+  - version: 2020.3
 ---
 {% for editor in test_editors %}
 test_mac_standalone_{{ editor.version }}:
     - pip install pyyaml --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple
     - python -u -m ml-agents.tests.yamato.standalone_build_tests
     - python -u -m ml-agents.tests.yamato.standalone_build_tests --scene=Assets/ML-Agents/Examples/Basic/Scenes/Basic.unity
-    - python -u -m ml-agents.tests.yamato.standalone_build_tests --scene=Assets/ML-Agents/Examples/Bouncer/Scenes/Bouncer.unity 
-    - python -u -m ml-agents.tests.yamato.standalone_build_tests --scene=Assets/ML-Agents/Examples/WallJump/Scenes/WallJump.unity 
+    - python -u -m ml-agents.tests.yamato.standalone_build_tests --scene=Assets/ML-Agents/Examples/Bouncer/Scenes/Bouncer.unity
+    - python -u -m ml-agents.tests.yamato.standalone_build_tests --scene=Assets/ML-Agents/Examples/WallJump/Scenes/WallJump.unity
   triggers:
     cancel_old_ci: true
     changes:

.yamato/training-int-tests.yml

 test_editors:
-  - version: 2018.4
+  - version: 2020.3
 ---
 {% for editor in test_editors %}
 test_mac_training_int_{{ editor.version }}: