Switch aria role heading to banner

[essjay05]

Updates the previous aria where it was meant to indicate the semantic html tag to its associated aria role since aria is typically used in conjunction with with a number of 1 through 6 to signify what heading level type of tag it's to be read as.

Fixes issue

Related to bug fix

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [ ] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] I signed the CLA.
• [ ] All existing and new tests are passing.

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