[18/n] add target release generation to planning input

[sunshowers]

Decided to put this inside tuf_repo to attach the two to each other. old_repo stays the same because the generation is implicit there.

Depends on:

• [17/n] use IdOrdMap to index sleds in a few spots #8400
• [16/n] send resolver status up via inventory #8300

[sunshowers]

old_repo stays the same because the generation is implicit there.

Actually, I don't think this is quite right in general, though it seems to be the logic for PlanningInputFromDb. Are we going to need the old repo's generation number, though? I'm not quite sure.