Update accessibility template v6.0.1

Ref: gitlab-org/ci-cd/accessibility#11 (closed)

What does this MR do and why?

This MR our accessibility ci template for pa11y to its latest version v6.0.1. Here the full diff.

In this major version bump the new scanner will report more errors as it now using WCAG 2.1 rules.

To mitigate the impact on our users we added a changelog entry to users are aware the new scanner is now more strict and we also added a note to our documentation.

Why are we doing this?

As new software versions get released it's important to stay up to date so to protect ourself from security vulnerabilities and ~"technical debt" accumulation.

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.