MacOS version requirements

Somewhere between the old downloads page and the new -- and with the dropping of Qt5 support, we've lost some clear advice for users on older versions of macOS. We should restore some of that to https://docs.ankiweb.net/platform/mac/installing.html#requirements --

> macOS version requirements are listed on the download page.
>
> If you’re on an old machine, you can obtain the old version from the [releases page](https://github.com/ankitects/anki/releases). The Qt5 builds on 24.11 and earlier support macOS 10.14 and later. If your macOS is between 10.10 and 10.13, you’ll need to use [Anki 2.1.35-alternate](https://github.com/ankitects/anki/releases/tag/2.1.35).

Or maybe that's everything and it could just use with some clarification. I just gave this advice, which I hope is "safe," but it's possibly not comprehensive: https://www.reddit.com/r/Anki/comments/1n3e365/older_version_of_anki/ .

From Wayback:

- Downloads: https://web.archive.org/web/20240529193933/https://apps.ankiweb.net/


- Downloads: https://web.archive.org/web/20241229103937/https://apps.ankiweb.net/


- Installing: https://web.archive.org/web/20240521013402/https://docs.ankiweb.net/platform/mac/installing.html#qt5-vs-qt6 [in case we want to offer a shorter version of this]