Introduce remediations to activity filter

What does this MR do and why?

Add "Solution" (Remediation) section to the Activity filter

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Have the related FF enabled: echo "Feature.enable(:activity_filter_has_remediations)" | 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 "Solution available" items
7. When selected, the vulnerabilities will be filtered accordingly

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 #427622 (closed)

config/feature_flags/development/activity_filter_has_remediations.yml

+---
+name: activity_filter_has_remediations
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/135009
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/429262
+milestone: '16.6'
+type: development
+group: group::threat insights
+default_enabled: false