Use packaged script from GitLab accessibility image

What does this MR do?

This replaces the bash script written directly in the a11y template with a call to a bash script file packaged in the new accessibility image.

This configuration template will produce an HTML accessibility report for each URL in $a11y_urls, as well as a single accessibility.json artifact that can be fetch and parsed programatically by frontend features.

This feature doesn't run in pipelines by default, even with the template included. a11y_urls must be specifically set in order for the job to run.

I set up a test project pulling the GitLab Accessibility CI template off this feature branch, and the job with artifacts is viewable here: https://gitlab.com/drewcimino/a11y-scanning/-/jobs/466028486. I'm not sure if there's a better way to test a CI template in our project setup, but this seems sufficient to me for the purposes of this MR.

The accessibility project that the image comes from can be found here: https://gitlab.com/gitlab-org/ci-cd/accessibility

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• ~~Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team