fix(alert): add default aria-label

[averyjohnston]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe): Accessibility-related fix

What is the current behavior?

ion-alert throws Axe accessibility violations by default due to not having an aria-label.

Issue URL: N/A (internal ticket)

What is the new behavior?

Default aria-label added, pulling from the alert's header (or subHeader if no header is defined) and falling back to simply 'Alert' if needed. Devs can already define a custom aria-label using the htmlAttributes property when presenting the alert.

Does this introduce a breaking change?

• Yes
• No

Other information