Update DependenciesPolicy.md

[dpino]

Remove entries for Ubuntu 20.04 and Debian 11 since they are no longer supported. Add a new section, 'Feature Support', with a schedule indicating until when a feature (e.g. libsoup2, gtk3) will be supported.

[mcatanzaro]

I'm OK with dropping the GTK 3 support in April 2027, but it does mean a whole lot of apps will stop receiving security updates. We should communicate this prominently. I also don't think this has actually been discussed or agreed on?

In contrast, removing the libsoup 2 support in April 2027 seems uncontroversial, since apps should have already ported to libsoup 3 by now. That's actually much longer than I would have expected it to be supported. (Certainly Ubuntu 24.04 already includes libsoup 3.)

[dpino]

Sorry, I think the chart doesn't communicate what I intended properly.

About libsoup2, the reason why it has to remain until April 2027 is because it's still featured in Ubuntu 24.04. Yes, it's true that Ubuntu 24.04 features libsoup3, but since also features libsoup2 there might be already applications that use webkitgtk + libsoup2 and they need to be supported (including security updates). OTOH, libsoup2 requires GTK3, so as long as libsoup2 needs to be supported, GTK3 must remain. So, in the case of GTK3 the deadline should be "at least until April 2027", but it could be longer.

Part or this conversation happened in a PR I opened to remove the libsoup2 code paths.