Revert "pin CI image version (#5250)" (#5285)

This reverts commit 3d8ad73968a7eb9401f055860410a2449da536b9.

.yamato/com.unity.ml-agents-optional-dep-tests.yml

     name : Test Optional Package Dependencies {{ optional_dep.name }}
     agent:
         type: Unity::VM
-        image: package-ci/ubuntu:v1.3.1-719011
+        image: package-ci/ubuntu:stable
         flavor: b1.medium
     commands:
         - |

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

   name: Pack
   agent:
     type: Unity::VM
-    image: package-ci/ubuntu:v1.3.1-719011
+    image: package-ci/ubuntu:stable
     flavor: b1.small
   commands:
     - |

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

       flavor: b1.small
     - name: linux
       type: Unity::VM
-      image: package-ci/ubuntu:v1.3.1-719011
+      image: package-ci/ubuntu:stable
       flavor: b1.medium
 
 packages:

.yamato/compressed-sensor-test.yml

   name: Test Compressed Sensor Observation {{ editor.version }}
   agent:
     type: Unity::VM
-    image: package-ci/ubuntu:v1.3.1-719011
+    image: package-ci/ubuntu:stable
     flavor: b1.medium
   variables:
     UNITY_VERSION: {{ editor.version }}

.yamato/coverage_tests.metafile

 coverage_test_platforms:
     - name: linux
       type: Unity::VM
-      image: package-ci/ubuntu:v1.3.1-719011
+      image: package-ci/ubuntu:stable
       flavor: b1.medium
 
 coverage_test_packages:

.yamato/gym-interface-test.yml

   name: Test Linux Gym Interface {{ editor.version }}
   agent:
     type: Unity::VM
-    image: package-ci/ubuntu:v1.3.1-719011
+    image: package-ci/ubuntu:stable
     flavor: b1.medium
   variables:
     UNITY_VERSION: {{ editor.version }}

.yamato/protobuf-generation-test.yml

   name: Protobuf Generation Tests
   agent:
     type: Unity::VM
-    image: package-ci/ubuntu:v1.3.1-719011
+    image: package-ci/ubuntu:stable
     flavor: b1.large
   variables:
     GRPC_VERSION: "1.14.1"

.yamato/pytest-gpu.yml

   name: Pytest GPU
   agent:
     type: Unity::VM::GPU
-    image: package-ci/ubuntu:v1.3.1-719011
+    image: package-ci/ubuntu:stable
     flavor: b1.large
   commands:
     - |

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

   name: Test Linux LL-API {{ editor.version }}
   agent:
     type: Unity::VM
-    image: package-ci/ubuntu:v1.3.1-719011
+    image: package-ci/ubuntu:stable
     flavor: b1.medium
   variables:
     UNITY_VERSION: {{ editor.version }}

.yamato/standalone-build-test.yml

   name: Test Linux Standalone {{ editor.version }}
   agent:
     type: Unity::VM
-    image: package-ci/ubuntu:v1.3.1-719011
+    image: package-ci/ubuntu:stable
     flavor: b1.large
   variables:
     UNITY_VERSION: {{ editor.version }}

.yamato/standalone-build-webgl-test.yml

   name: Test WebGL Standalone {{ editor_version }}
   agent:
     type: Unity::VM
-    image: package-ci/ubuntu:v1.3.1-719011
+    image: package-ci/ubuntu:stable
     flavor: b1.large
   variables:
     UNITY_VERSION: {{ editor_version }}

.yamato/training-int-tests.yml

   name: Test Linux Fast Training {{ editor.version }}
   agent:
     type: Unity::VM
-    image: package-ci/ubuntu:v1.3.1-719011
+    image: package-ci/ubuntu:stable
     flavor: b1.medium
   variables:
     UNITY_VERSION: {{ editor.version }}