[Resolve This Vulnerability] - Clean up CreateFromVulnerabilityDataService to no longer rely on `finding.metadata`

As a followup to the following comment: !133123 (comment 1633137239)

The implementation of Resolve This Vulnerability relies on the MergeRequest/CreateFromVulnerabilityDataService initially implemented for the existing remediation strategies in GitLab. However this service uses the finding.metadata method which is deprecated and intended to be removed. As such we would like to modify this service's use and implementation to not depend on that method and rather take the information from the underlying security_finding as needed.

Implementation Plan

• Reconfigure MergeRequest/CreateFromVulnerabilityDataService to take a vulnerability object directly, modifying it's usage in GitLab to cease reliance on finding.metadata