Backend: Add hasMergeRequest to Severities Count

Let's add hasMergeRequest to the vulnerabilityeSeveritiesCount. Here's hasIssues for reference:

hasIssues (reference)	hasMergeRequest (current)

It is needed to update the counts. Here's an example with hasIssues

Implementation plan:

• Add has_merge_request argument to ee/app/graphql/resolvers/vulnerability_severities_count_resolver.rb (see #426230 (comment 1582522782))