Rolling tags for upstream stable tracking

Goal: Track official upstream designations for the various non-EoL majors at any given point in time

Existing but better document:

• stable always point to the highest numbered of the currently supported majors (e.g 27, 28, 29) that is published to 100% of instances via the Nextcloud Update Server.
  • Trackable here
• latest always points at the absolute highest of the currently supported majors without regard for 100% general availability.
  • On the surface this seems like an easy one, but this is an oddball because it's also equivalent to not specifying any label at all and therefore the Docker default. Since it's the de facto entry point for newcomers, it is reasonably debatable whether starting people on the most bleeding edge is always ideal.
  • Outside of the Apache/default, latest is also a modified for the other image types (fpm, alpine) in the same manner
• production is just a mirror of stable.

New (proposed not actually implemented):

• previous is the most recent major behind the latest major which is receiving regular bug fixes.
• last is the oldest major still receiving updates.

Related:

• {major}-{apache, fpm} (e.g. 28-apache) always points to the latest published maintenance release for the specified major
• *-alpine variations in all cases where applicable (no need for debian/bookworm since that's our default base)

No implementation file or test is named; start by reviewing the documented stable, latest, production, previous, and last semantics alongside the linked updater_server pull requests. Trace how this repository publishes Docker tags, including apache, fpm, and alpine variants, and consider the work done when the proposed rolling tags consistently match the stated upstream support and availability rules.