Add dismissal reason descriptions popover to findings modal

What does this MR do and why?

Related #422845 (closed)

This adds a help icon with popover next to the Dismiss as label which shows all the dismissal reasons and their descriptions. This assists the user in choosing the correct dismissal reason when dismissing a finding.

For more context, this thread summarizes the design decision to go for this popover which best supports touch, keyboard, and mouse.

Screenshots or screen recordings

How to set up and validate locally

Prerequisites

1. You need an EE license
2. You need to have runners enabled (See $2408961 for setting up a runner)
3. Import https://gitlab.com/gitlab-examples/security/security-reports
4. Run a pipeline on master

Validation

1. Go to a pipeline security tab, like e.g.: http://gdk.test:3000/root/security-reports/-/pipelines/1/security?severity=LOW
2. Select the "More info" info icon of a non-dismissed finding
3. Click Dismiss vulnerability in the modal
4. Hover, click, or tab to the Help popover next to the Dismiss as label
5. Validate it opens a popover with the descriptions of all dismissal reasons.

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.