Compatibility table update

[marbru]

I recently installed the app in ubuntu 24.04 using the deb package. However, I had a hard time deciphering the compatibility table. From the discussions, seems like I'm not the only one confused by the version that should be used (eg #642 (reply in thread)).

This PR attempts to clean the table a little bit:

• simplify version lists: ubuntu bionic now lists simply "18.04 and newer", plus the text below on why there are two ubuntu versions is more understandable
• remove non-existing packages: disco, eoan, and buster don't seem to exist, and indeed were striked in the table. Removed so that they don't add to the confusion
• fix system build for rpm systems in the table, which was wrong
• remove trailing ul of distributions that were totally out of context (probably leftover from previous edits?)
• remove info about the PPAs, since they are now discontinued
• deduplicate redundant paragraph about AUR package
• deduplicate redundant paragraph about compatibility of similar systems (now at the end)

Hopefully, this makes things easier to follow.