V4 dev backports and updates

[XhmikosR]

Refs #31823

[XhmikosR]

This should restore the ES5 enforcement in tests @Johann-S

[XhmikosR]

I left js/dist.index.js for the time being, although it does seem unused. We should revisit later if we need it and remove it if not.

[mdo]

I left js/dist.index.js for the time being, although it does seem unused. We should revisit later if we need it and remove it if not.

I see this is covered in #31823, so maybe we slate for v4.6.0? Do we need to warn folks at all?

[XhmikosR]

I see this is covered in #31823, so maybe we slate for v4.6.0? Do we need to warn folks at all?

I intentionally kept index.js in v4-dev, since I can't be 100% sure if this will break something. But the sourcemap was outdated and moot for sure, hence why I decided to remove only that here :)