Fix instance level dashboard by default severity override

What does this MR do and why?

This fixes the instance-level vulnerability report dashboard which does not show vulnerabilities. It can be found at the path: /-/security/vulnerabilities.

The problem is we're passing a feature flag as a variable for an Apollo GraphQL query which needs to be a boolean. The feature flag is not pushed to the frontend for the instance-level so it passes undefined and this causes a mismatch with Apollo.

This MR fixes it by providing default value of false when the feature flag value is undefined.

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes). !182112 (merged)
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).