Version menu is getting too long

[cbrnr]

Somewhat related to #10789, our version menu is getting pretty long (and if we don't do anything about it, it will get too long rather soon):

I propose that in addition to devel and stable, we only show five previous versions and then maybe a link to a page with older versions. Alternatively, we might just drop older versions altogether – or is there a particular reason why someone would want to use the documentation for MNE-Python that's over 3 years old?

[larsoner]

Another option might be a little CSS fix like

.whatever_drop_down
{
   height: 50px;
   overflow: auto;
}

I'm also okay with making the list shorter. Happy to defer to @drammock on this

[larsoner]

(the above or some similar variant would add a scroll bar to the dropdown)

[cbrnr]

A scrollbar would also work, but at some point it too will get unwieldy. I'm fine with a short-term and medium-term solution though.