Drop support for using axe-core in Internet Explorer

[WilcoFiers]

IE11 is no longer supported by Microsoft, many dependencies we use no longer support it. It's time to let go. In the next version of axe-core, IE will no longer be supported. The main thing this means is:

1. Documentation needs to be updated
2. IE tests will no longer run in CI
3. Most, if not all polyfills can be pulled.
4. There's a color parser somewhere that was only created for IE which I think we can pull
5. We can remove all the axe.testUtils.isIE11 things in our tests

There may be a bunch of IE specific code paths. Pulling those out is going to take a while, we'll probably do that gradually, rather than as one big chunk. Those are going to be difficult to find.

important: This is different from us dropping IE11 as an accessibility supported browser (see #3474). This ticket is specifically about dropping support for running axe-core in IE11, it is unrelated to testing for accessibility issues that only occur in JAWS+IE.

[straker]

This is a breaking change. Even removing testing in IE11 has the potential to introduce breaking bugs into IE11, so we shouldn't do that either. However, what we should do in 4.5 is deprecate IE11 support. Maybe by updating the Accessibility Supported document to say that IE11 support is now deprecated?

[WilcoFiers]

Dropping support for unmaintained projects is not a breaking change IMO. Our backward compabitiiliby doc doesn't list this as a breaking change.

[WilcoFiers]

So conclusion, we're deprecating support for IE11 as of axe-core 4.5. It will be fully removed as of 5.0. With that deprecation though, we'll also stop running automated tests against IE11. Anyone experiencing issues in using axe-core in IE11 can still report them, so we can investigate.