[popover2] feat(Popover2, Tooltip2): make component type param optional

[adidahiya]

Changes proposed in this pull request:

It turns out #5713 also had a typings regression for the popoverRef prop on datetime2 & select components, specifically the bit with React.HTMLProps<unknown>. In trying to improve this type definition, I found myself writing React.HTMLProps<HtmlElement> in a lot of places, since that's the default assumed in the definition of Popover2Props<T = ...>. It seems like we can actually just define this as the default in the Popover2 class definition and avoid specifying the type param in most cases. I'm not exactly sure why this wasn't the case before (T was required if referencing Popover2 as a type), but this will make these type references a lot simpler in our codebase and for consumers.

[adidahiya]

Ok, it looks like we can't actually do this. Providing a default value for the Popover2<T> type param makes it hard to deal with the target HTML props interface in renderTarget, leading to issues like this:

... which might be easily solved by using <Popover2<React.HTMLProps<HTMLButtonElement> in the opening tag of that code example, but we don't want to introduce a type regression which forces all consumers to instantiate the type of their Popover2s.

Opening an alternative PR to fix the type regression: #5735