This change allows Discussions::ResolveService to infer the related
Merge Request without the params[:merge_request] being present.

Previously the param was expected to be passed and the service would
error if it were not present.

This change is being made for two reasons:

1) Discussions can be associated with Designs, which have no
relationship with Merge Requests. In order to be able to use this
service to resolve Discussions on Designs, we need to make this service
able to treat the param as optional.

2) This service is to be called from within the GraphQL API. It
shouldn't be the API's responsibility to find the related Merge Request
and pass it to the service, but instead the service should be
responsible for making sure the right services are called after
resolving Discussions on Merge Requests.

The params[:merge_request] has been kept as an optimisation for existing
code that has fetched the Merge Request already before calling the
service (this won't be the case for the GraphQL API).

https://gitlab.com/gitlab-org/gitlab/-/issues/13049