Remove deprecated a11y-no-onchange warning

[geoffrich]

Removes the a11y-no-onchange compiler warning. See #4946 (comment) for the rationale behind this change. tl;dr: the jsx-a11y rule this warning was based on is now deprecated. The original rationale is from 2004 and no longer an a11y best practice for modern browsers (or even IE11).

My comment on the original issue thread received 20+ positive reactions since I posted it in January, so figured I'd submit a PR to get the ball rolling. If this change is not wanted, feel free to close.

Fixes #4946

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[Rich-Harris]

nice, thank you!

[Conduitry]

Released in 3.39.0 - thanks!