Migrate sourcesecret package to runtime/secrets APIs

The sourcesecret package now uses pkg/runtime/secrets factory
functions instead of the previous monolithic approach. This
provides standardized secret generation with consistent
validation and error handling across all authentication types.

Signed-off-by: cappyzawa <cappyzawa@gmail.com>

cmd/flux/testdata/create_secret/oci/create-secret.yaml

@@ -5,6 +5,15 @@ metadata:
   name: ghcr
   namespace: my-namespace
 stringData:
-  .dockerconfigjson: '{"auths":{"ghcr.io":{"username":"stefanprodan","password":"password","auth":"c3RlZmFucHJvZGFuOnBhc3N3b3Jk"}}}'
+  .dockerconfigjson: |-
+    {
+      "auths": {
+        "ghcr.io": {
+          "username": "stefanprodan",
+          "password": "password",
+          "auth": "c3RlZmFucHJvZGFuOnBhc3N3b3Jk"
+        }
+      }
+    }
 type: kubernetes.io/dockerconfigjson