Fix dependency parsing errors caused by collision_check option (#3760)

A new option, `collision_check`, was added to the `deploy` task. The new
option wasn't added to `core/dependencies.py`, so attempting to
deserialize resulted in a DependencyParseError in MetaDeploy.

cumulusci/core/dependencies/dependencies.py

@@ -544,6 +544,7 @@ class UnmanagedDependency(StaticDependency, abc.ABC):
     subfolder: Optional[str] = None
     namespace_inject: Optional[str] = None
     namespace_strip: Optional[str] = None
+    collision_check: Optional[bool] = None
 
     def _get_unmanaged(self, org: OrgConfig):
         if self.unmanaged is None:

cumulusci/core/dependencies/tests/test_dependencies.py

@@ -903,3 +903,13 @@ def test_parse_unmanaged_dependency(self):
             }
         )
         assert isinstance(u, UnmanagedZipURLDependency)
+
+        u = parse_dependency(
+            {
+                "github": "https://github.com/Test/TestRepo",
+                "ref": "aaaaaaaa",
+                "collision_check": False,
+                "namespace_inject": "ns",
+            }
+        )
+        assert isinstance(u, UnmanagedGitHubRefDependency)