Add hasMergeRequest to activity filter

What does this MR do and why?

This MR adds the "hasMergeRequest" to the Activity Filter

🚨 Note: the "severity counts" section doesn't currently work, it will be addressed in a different MR

Screenshots or screen recordings

FF	Project	Group	Security Control (Instance)
Enable
Disable

How to set up and validate locally

1. Have the related FF enabled: echo "Feature.enable(:activity_filter_has_mr)" | rails c
2. In order to run a pipeline to generate a Vulnerability report, you'll need an EE license.
3. Fork https://gitlab.com/gitlab-org/govern/demos/sandbox/minac/test-remediations.
4. Run a pipeline against the default branch. It will generate vulnerabilities.
5. Go to the project vulnerability report page
6. Select an "Activity" filter, it will have the "Merge Request" items
7. When selected, the vulnerabilities will be filtered accordingly

To create MR on a vulnerability:

1. Click on a vulnerability
2. "Resolve with merge request"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #424649 (closed)