Fix endpoint URI format in workflow SCHEMA

[ricardozanini]

What seems off:
The Workflow Schema uses the uri format in the endpoint property. This format is not valid for template URI like https://petstore.swagger.io/v2/pet/{petId}

What you expected to be:
That either URI and URI template passes the schema tests. See https://json-schema.org/understanding-json-schema/reference/string#resource-identifiers

Anything else we need to know?:
See GHA failure from #865 PR.

Environment:

• Specification version used:
  1.0.0-alpha1