Configuration of new test environment and next steps for plone.org upgrade

[fredvd]

This issue attempts to be a summary of results so far and ideas/steps to get the plone.org upgrade project further.

• @sneridagh made a vlt branch where he updated the frontend and the frontend scaffold to the latest Volto 18, VLT and cookieplone from November 2024. @davisagli updated these versions further
• For tagung.plone.de and 2025.ploneconf.org @fredvd has new iterations for the ci/cd workflow to the plone foundation cluster.
• The current backend is Plone 6.0.15, but not with the latest/greatest cookieplone uv and formatting/linting setup.
• during the Axelotl sprint, we would like to get a 'testing' environment where we can share work to upgrade the site.

Upgrade detail discussions:

• the site has been built with community provided add'ons/blocks in 2022/early 2023, and custom styling, and hasn't had much love/update since, except for fixes and small additions. The whole idea of moving to VLT is to reduce 'maintenance styling effort, while keeping somewhat aligned with the current look. But for now we do not want to rebuild a whole custom theme yet:
• A number of used (community) blocks still use draftjs. These need to be migrated to slate, if we want to keep them.
• A number of custom blocks that are only in use a few times, can maybe be removed and replace by other (community provided) blocks.
• 'adapting' the current css to a foundation with VLT will probably be more work and result in lots of patches and frankencss. But we don't know yet. Listing templates have for example been customised, these need to be done again.
• To choose the right direction later, we first need a 'bare bones' volto 18 / vlt setup first that renders most of the pages,
  • so that we plan how to upgrade missing blocks
  • see the migration effort
  • decide how much styling and which blocks we we want
  • and lastly if that styling should be close to the current site or should be refreshed more as well

I suggest we keep working on the VLT branch as a long lived branch for the next weeks:

• I will create next.plone.org where this branch does CD to.
• we need to update the backend scaffold part as well to benefit from improved formatting/linting and image build
• we know the cookieplone scaffold quite good, bigger changes between november 24 and may 25 have been in the backend, the frontend might need a scan and some smaller updates. @sneridagh ?
• I can then also re-use 2025.ploneconf.org and another setup for CI/CD, which are all based on cookieplone.