Remove `clusters` from scan execution policy schema

Why are we doing this work

Our security policy schema specifies the clusters property. This property was used by the certificate-based Kubernetes integration, which was deprecated in %14.5. the Cluster Image Scanning Analyzer, which was removed in %15.0 (#356465 (closed)).

We only kept clusters around for backwards compatibility: !87424 (comment 946442589). The agent-based integration uses agents, so we're good to remove clusters.

Implementation Plan

diff --git a/ee/app/validators/json_schemas/security_orchestration_policy.json b/ee/app/validators/json_schemas/security_orchestration_policy.json
index d99e98ebe06e..f6799bc766fc 100644
--- a/ee/app/validators/json_schemas/security_orchestration_policy.json
+++ b/ee/app/validators/json_schemas/security_orchestration_policy.json
@@ -128,43 +128,6 @@
                       }
                     }
                   }
-                },
-                "clusters": {
-                  "type": "object",
-                  "minProperties": 1,
-                  "maxProperties": 1,
-                  "additionalProperties": false,
-                  "patternProperties": {
-                    "^[a-z0-9]([-a-z0-9]*[a-z0-9])?$": {
-                      "type": "object",
-                      "properties": {
-                        "namespaces": {
-                          "type": "array",
-                          "items": {
-                            "type": "string"
-                          }
-                        },
-                        "resources": {
-                          "type": "array",
-                          "items": {
-                            "type": "string"
-                          }
-                        },
-                        "containers": {
-                          "type": "array",
-                          "items": {
-                            "type": "string"
-                          }
-                        },
-                        "kinds": {
-                          "type": "array",
-                          "items": {
-                            "type": "string"
-                          }
-                        }
-                      }
-                    }
-                  }
                 }
               },
               "if": {