Add logic to handle dismissal reason

What does this MR do and why?

As part of Vulnerability Dismissal Types / Reasons (&4942 - closed) when users choose to dismiss a vulnerability they will be prompted to choose a reason.
We need to update the E2E tests to handle this new flow as we need to choose a reason when we choose the 'dismiss' option.

This test handles this flow when the dismissal_reason feature flag is enabled, but should also support running when the FF is not enabled.

Fixes Failure in ee/browser_ui/10_govern/change_vulne... (#406657 - closed)

How to set up and validate locally

• bundle exec rails console
• Feature.enable(:dismissal_reason) / Feature.disbale(:dismissal_reason)
• cd /path/to/gdk/gitlab/qa
• bundle exec rspec ./qa/specs/features/ee/browser_ui/10_govern/change_vulnerability_status_spec.rb
• Repeat running the spec multiple times to ensure that the 'dismiss' flow is triggered especially with Feature.enable(:dismissal_reason)

MR acceptance checklist

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