xk6 topic should not be a requirement for internal extensions

[szkiba]

The internal linter currently expects the xk6 topic to be set on the extensions repository. In the case of internal (but public) extensions, it would not be a good idea to use the xk6 topic, because that would cause them to appear in the GitHub topic search.

The internal linter should not expect the xk6 topic to be set for extensions that do not have oss in their products property. That is, in the case of extensions that cannot be used in Grafana k6, the use of the xk6 topic should not be required.