[web_ui] Fixed aria-live attribute on web

[TheOneWithTheBraid]

This PR fixes the incorrectly set aria-live attribute in the web's semantics. Up to now, it has been set to true, which is an invalid value. According to MDN, it should be set to polite, assertive or off.

Moreover, several typos in the semantics test have been fixed.

List which issues are fixed by this PR. You must list at least one issue.

• Web incorrectly sets aria-live attribute flutter#87939

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

• I don't think so.

Please note, there was an existing test I modified. Unfortunately, this test was expecting the incorrect value.

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 Hixie said 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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[yjbanov]