Make GlButton accessible while loading

What does this MR do?

In Add prop to keep GlButton accessible when loading (!4979 - merged) • Rahul Chanila • 18.5 we added a prop to make the loading button focusable.

We decided to make this configurable via a feature flag and so we added support for the config in the above MR.

In Allow setting accessibleLoadingButton config fo... (gitlab-org/gitlab!208901 - merged) we added the config in gitlab-org/gitlab project to be controlled via a feature flag.

The feature flag has been enabled gitlab-org/gitlab#577222 (comment 3004166132) for couple of weeks now and for a couple of months for all GitLab team members and we haven't seen any incidents.

We can now make this the default button loading behaviour and remove accessibleLoading button config.

Screenshots or screen recordings

Integrations

• GitLab: Draft: GitLab integration test for https://gitl... (gitlab-org/gitlab!218730 - closed)
  • gitlab-org/gitlab!218730 (a7219584) should fix the failing tests
• CustomersDot: mr_url
• Duo UI: mr_url
• Status Page: mr_url
• Docs: mr_url
• Switchboard: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

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• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

Related to #2981 (closed)