[yamato] drop 2020.1, reduce test variants, tighten triggers (#4936)

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

 test_editors:
   - version: 2019.4
-  - version: 2020.1
   - version: 2020.2
 ---
 {% for editor in test_editors %}

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

     enableCodeCoverage: !!bool true
     testProject: DevProject
     enableNoDefaultPackages: !!bool true
-  - version: 2020.1
-    enableCodeCoverage: !!bool true
-    testProject: DevProject
-    enableNoDefaultPackages: !!bool true
   - version: 2020.2
     enableCodeCoverage: !!bool true
     testProject: DevProject

.yamato/compressed-sensor-test.yml

     - .yamato/standalone-build-test.yml#test_linux_standalone_{{ editor.version }}
   triggers:
     cancel_old_ci: true
+    {% if editor.extra_test == "sensor" %}
     expression: |
       (pull_request.target eq "master" OR
       pull_request.target match "release.+") AND
       pull_request.changes.any match "Project/**" OR
-      pull_request.changes.any match "ml-agents/**" OR
+      pull_request.changes.any match "ml-agents/tests/yamato/**" OR
+    {% endif %}
 {% endfor %}

.yamato/gym-interface-test.yml

     - .yamato/standalone-build-test.yml#test_linux_standalone_{{ editor.version }}
   triggers:
     cancel_old_ci: true
+    {% if editor.extra_test == "gym" %}
     expression: |
       (pull_request.target eq "master" OR
       pull_request.target match "release.+") AND
-      pull_request.changes.any match "ml-agents/**" OR
+      pull_request.changes.any match "ml-agents/tests/yamato/**" OR
+    {% endif %}
 {% endfor %}

.yamato/python-ll-api-test.yml

     - .yamato/standalone-build-test.yml#test_linux_standalone_{{ editor.version }}
   triggers:
     cancel_old_ci: true
+    {% if editor.extra_test == "llapi" %}
     expression: |
       (pull_request.target eq "master" OR
       pull_request.target match "release.+") AND
-      pull_request.changes.any match "ml-agents/**" OR
+      pull_request.changes.any match "ml-agents/tests/yamato/**" OR
+    {% endif %}
 {% endfor %}

.yamato/test_versions.metafile

 # List of editor versions for standalone-build-test and its dependencies.
-# csharp_backcompat_version is used in training-int-tests to determine the
-# older package version to run the backwards compat tests against.
+# We always run training-int-tests for all versions of the editor
+# For each "other" test, we only run it against a single version of the
+# editor to reduce the number of yamato jobs
-    csharp_backcompat_version: 1.0.0
+    extra_test: llapi
-    csharp_backcompat_version: 1.0.0
-  - version: 2020.1
-    csharp_backcompat_version: 1.0.0
+    extra_test: gym
-    # 2020.2 moved the AssetImporters namespace
-    # but we didn't handle this until 1.2.0
-    csharp_backcompat_version: 1.2.0
+    extra_test: sensor