On Java 8 and Scala 2.12

[djspiewak]

Over the past six months in particular, I've noticed an accelerating trend from various elements of the ecosystem to abandon support for or even outright prevent these older versions. Java 8 is a particular stone in the shoe because of Sbt 2 (which requires a minimum version of Java 11 Java 17), but Scala 2.12 is also relevant.

Going off of very little other than general vibes and impatience, I've decided that I would like to move both of these into the same category we placed Scala 2.11 way back when the last time we had to deprecate a supported version. Which is to say: I'm not going to actively remove support tomorrow, but the moment I need to work particularly hard to maintain support, I'm going to rip it out. Or, phrased another way: the yak-shave annoyance threshold at which deletion is preferred is now much lower.

We would not make this change in a patch release; it would be a minor bump (e.g. 3.8.0 or 3.9.0 or something like that), and again, I'm not saying we're doing this tomorrow, I'm just saying that I think we should do it next time we hit a meaningful hurdle in maintenance which stems from either (or both) of these versions. In practice, what this means is that if you're stuck on one or both of these versions when we rip the bandaid, you won't be able to update to a newer major/minor Cats Effect version. We do generally have the ability to release bug and security fixes against arbitrarily old major/minor lineages (aside from the perennial issues with infrastructural bitrot), so you shouldn't consider yourself abandoned, but you would be strongly encouraged to upgrade as soon as possible.

I'm open to being convinced otherwise, but consider the bar modestly high.

[mzuehlke]

sbt 2.x requires JDK 17 or above