Allow setting accessibleDisabledButton config for gitlab-ui

What does this MR do and why?

Allow setting accessibleDisabledButton config for gitlab-ui.

Adds the accessible_disabled_button feature flag and wires its value through gon.features.accessibleDisabledButton into applyGitLabUIConfig, so GlButton's new accessibleDisabled default can be controlled per environment during rollout.

Mirrors the pattern established for accessibleLoadingButton in !208901 (merged).

The feature flag defaults to off. No behavior change until enabled.

References

• Depends on !236541 (merged) (@gitlab/ui 134.1.1 bump)
• Rollout issue: #600158
• Upstream design feature: gitlab-org/gitlab-services/design.gitlab.com!5912 (merged)

How to set up and validate locally

Once !236541 (merged) is merged:

1. Enable the feature flag in a Rails console Feature.enable(:accessible_disabled_button)
2. Render any disabled GlButton. The button should now use aria-disabled="true" instead of the native disabled attribute, remain focusable, and be announced by screen readers.

MR acceptance checklist

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Related to #600158