Base Ubuntu version changing

I know `cimg/base` normally tracks relatively recent stuff on some kind of predictable cadence. That said, is there a way to get a _newer_ node tag (e.g., 14.21) but with the older Ubuntu version, via another tag (I didn't see one available)?

I realize it may not be possible, but IMO, it would be nice if these versions could stay predictable within a set of versions (i.e., when going from 14.20 to 14.21), or at least be accessed via a tag like `14.21-buster`.

(issue prompting me to open this: when I bumped our default tag from 14.20 to 14.21, we no longer had OpenSSL 1.1 available from the bump to jammy, which meant some stuff broke in our builds that I had to resolve with a kind of ugly hack).

Review the cimg/base and cimg-node tag relationship, especially Node 14.20, 14.21, and the proposed 14.21-buster tag, along with the Ubuntu and OpenSSL differences described in the report. Determine whether predictable Ubuntu variants can be published and define the supported tag behavior before making a change.

Written by the indexing model from the issue text.