Merge ~bryce/+git/server-backports:update-to-lunar into ~canonical-server/+git/server-backports:main

I've completed the backport from lunar to jammy (I think - please doublecheck!) This branch MP includes both the backport recipe updates themselves, and the updates I did to the Readme to both clarify the process I used and parameterize it a bit. Hopefully having this all in a unified MP will lend itself to easier review than split apart.

I got hung up a bit on the presence or absence of branches seemingly at random, until it was pointed out the missing ones were intentionally missing since they had no backport changes. I've attempted to call this out so it hopefully won't be a point of confusion for others in the future. Otherwise, the longer this PPA is maintained and the more branches come and go, the less obvious the state of the repository will be.

Another point of confusion came from the specified release codenames, since from one cycle to the next a release that was backported *from* will become one that is backported *to*. I've replaced those codenames with variables which both parameterizes the command lines and also gives them a name that communicates what they represent.

Another motivation to parameterize the examples was to make the document closer to paint-by-numbers style, which can help reproducibility and reduction of errors. Obviously, this also makes a firm step towards scripting some or all of the process, but I don't think I'm ready to take that step yet since there's still idiosyncrasies and thoughtful attention needed to spot and fix errors.

I've also introduced a `backports.json` file as discussed for capturing notes about each of the packages. I went with JSON since it is tool-friendly, and has a strongly typed data structure that should be non-ambiguous in how to maintain and extend it later. That said, consider both the format and structure to still be provisional, and may evolve as more data needs included and/or tools start making direct use of it.