Safekeeper: fix OpenAPI spec (#9066)

## Problem

Safekeeper's OpenAPI spec is incorrect:

```
Semantic error at paths./v1/tenant/{tenant_id}/timeline/{timeline_id}.get.responses.404.content.application/json.schema.$ref
$refs must reference a valid location in the document
Jump to line 126
```
Checked on https://editor.swagger.io

## Summary of changes
- Add `NotFoundError` 
- Add `description` and `license` fields to make Cloud OpenAPI spec
linter happy

safekeeper/src/http/openapi_spec.yaml

@@ -1,7 +1,11 @@
 openapi: "3.0.2"
 info:
   title: Safekeeper control API
+  description: Neon Safekeeper API
   version: "1.0"
+  license:
+    name: "Apache"
+    url: https://github.com/neondatabase/neon/blob/main/LICENSE
 
 
 servers:
@@ -386,6 +390,12 @@ components:
         msg:
           type: string
 
+    NotFoundError:
+      type: object
+      properties:
+        msg:
+          type: string
+
   responses:
 
     #