Download: Simplified download page, supports dual version downloads

[HermitShamiko]

@mgol @timmywil
The jQuery team released version 4.0 at the beginning of the year, but support for version 3.0 is not yet fully exhausted, and version 3.8.0 is expected to be released sometime later this year. Therefore, I was concerned that the download page might offer two different major versions simultaneously when version 3.8.0 is released. So, I optimized the layout of the download page to better provide downloads of both versions for a period of time.

Since version 3.x will soon be officially discontinued, I used a small amount of CSS in Markdown so that it can be easily removed when support ends.

I also referenced jquery/jquery#5787, which uses Maintenance and Limited maintenance to differentiate versions.
new:

old:

[timmywil]

Thank you for contributing. However, jQuery has historically only ever supported the latest version and that is still true for the most part, especially when it comes to the download page. Sometimes security issues have come in and we have made exceptions and issued a patch on the previous branch. So, at some point we started saying that we may provide updates for "critical issues" on a previous version. So, I will be updating the table you referenced to say that 3.x has "Critical-only" support, which you can read as "Limited" or "Maintenance only", "Legacy", etc.

In this case, we do have a 3.8.0 coming, but it's only to tie up loose ends and release the things that were already merged to the 3.x branch before 4.0 was released. It does not mean 3.x support has increased or that we encourage usage of 3.x.

Ultimately, we want to encourage everyone to upgrade to 4.0. To me, 3.x having its own section on the download page sends the wrong message. There is another page that describes our Version Support, and you can find downloads for 3.x if you really need them, but the only reason I see to use 3.x is if you need to support old browsers.

Also, FWIW, I think there was some confusion about the terms that were suggested in jquery/jquery#5787. "Maintenance" has special meaning. It was suggested as one of the options to describe 3.x, along with "Limited Maintenance', "Critical-only", and "Legacy". None of these work for 4.x, which has full support.