feat(GlBroadcastMessage): Add hardcoded sr-only heading

What does this MR do?

All the context in #1736 (comment 1360407689)

This MR add's a hardcoded screen ready only h2 to the broadcast message component. Having a heading will increase discoverability for assistive technologies like screen readers.

Although not yet used in GitLab, this makes the component ready for a future migration.

Follow up issues will implement this component in GitLab, and produce design specs for a user generated heading with optional visibility. 👣

Type = broadcast

Before	After
Visual:	Visual:
VoiceOver: No headings in VoiceOver rotor.	VoiceOver:

Type = notification

Before	After
Visual:	Visual:
VoiceOver: No headings in VoiceOver rotor.	VoiceOver:

Steps to test

1. Go to base > broadcast message > default http://localhost:9001/?path=/story/base-broadcast-message--default
2. View the page using a screenreader. Example instructions shown for VoiceOver, for more details see the VoiceOver user guide.
   1. Turn on VoiceOver.
   2. Ensure the component iframe has focus.
   3. See the list of headings using the VoiceOver rotor. ctrl+alt+u
   4. Check the Admin message h2 is visible.
3. Repeat for the notification and stacked stories.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

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
• Security reports checked/validated by a reviewer from the AppSec team

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.

Closes #1736 (closed)