Add missing 404 response to Swagger (#27038)

Most middleware throw a 404 in case something is not found e.g. a Repo
that is not existing. But most API endpoints don't include the 404
response in their documentation. This PR changes this.

routers/api/v1/user/gpg_key.go

@@ -63,6 +63,8 @@ func ListGPGKeys(ctx *context.APIContext) {
 	// responses:
 	//   "200":
 	//     "$ref": "#/responses/GPGKeyList"
+	//   "404":
+	//     "$ref": "#/responses/notFound"
 
 	listGPGKeys(ctx, ctx.ContextUser.ID, utils.GetListOptions(ctx))
 }