fix(alert): use aria-labelledby and aria-describedby instead of 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 fixes

What is the current behavior?

In #25800, we added a default aria-label to ion-alert in order to resolve an Axe violation. However, there are two problems with this resolution:

1. The default aria-label of 'Alert' isn't any better than having no label at all, as it doesn't give any useful information. If anything, it can lead developers into a false sense of security due to the lack of Axe errors.
2. The ARIA spec mentions that the alertdialog role must also have an aria-describedby attribute.

Issue URL: N/A

What is the new behavior?

• aria-label replaced with aria-labelledby and aria-describedby. The former points to the header (or the subHeader if no header is defined), and the latter points to the message.
• If the required properties aren't set for either attribute (for example, no message), the attribute is left unset.

Does this introduce a breaking change?

• Yes
• No

Other information

A separate PR will be put up to document this change and show best practices with accessibility.