Supported browser versions

We should be explicit about which browser versions we support, and take steps to ensure we remain compatible.

Currently we state on our website that we are compatible with Chrome, Firefox, and Brave, but we don't specify any minimum required browser versions. Opera is listed as well, but that version is very outdated. We don't regularly test which browsers the application works on either, relying instead upon bug reports for compatibility with older browsers.

There was a previous issue for documenting which browsers we support (#3343), but that was closed with an adjustment of our Babel configuration file to support all browser versions with at least 0.25% global usage (according to `browserlist`), excluding IE 11 and Opera Mini. However, this includes some _very old_ browser versions, more than we'd care to support in practice.

Instead we should document explicitly which browsers we support, and add tests to ensure we stay compatible (perhaps to run nightly, so we don't overly burden CI). Then we can get rid of the Babel transformations and polyfills we no longer need. We should also update the manifests to state the minimum browser version, so users don't accidentally install the extension on an unsupported browser.

Steps:

- [x] Choose minimum supported browser versions
- [ ] Document the supported browser versions
- [x] Remove Babel transformations and polyfills used for compatibility with unsupported browsers
- [ ] Add nightly tests for minimum supported browser versions
- [x] Update manifests to include minimum browser version