Fix incorrect default on readonly property in our API

The new version of openapi-spec-validator (0.6.0) started to
detect a bug in our API specification - readonly values cannot
have defaults (by definition).

This PR fixes the problem and also bumps the openapi-spec-validator
to >= 0.6.0 to make sure it is used everywhere.

.pre-commit-config.yaml

@@ -317,7 +317,7 @@ repos:
         name: Lint OpenAPI using openapi-spec-validator
         entry: openapi-spec-validator --schema 3.0.0
         language: python
-        additional_dependencies: ['openapi-spec-validator']
+        additional_dependencies: ['openapi-spec-validator>=0.6.0']
         files: ^airflow/api_connexion/openapi/
       - id: lint-dockerfile
         name: Lint Dockerfile

airflow/api_connexion/openapi/v1.yaml

@@ -2892,7 +2892,6 @@ components:
           $ref: '#/components/schemas/DagState'
         external_trigger:
           type: boolean
-          default: true
           readOnly: true
         conf:
           type: object

airflow/www/static/js/types/api-generated.ts

@@ -1076,7 +1076,6 @@ export interface components {
       /** @enum {string} */
       run_type?: "backfill" | "manual" | "scheduled" | "dataset_triggered";
       state?: components["schemas"]["DagState"];
-      /** @default true */
       external_trigger?: boolean;
       /**
        * @description JSON object describing additional configuration parameters.