Upgrade symfony 5

What does this MR do?

Upgrades to symfony 5, also updates some dependent php packages. Changes symfony to be included via "^5.1" version constraint, as we did not include all symfony packages directly anyway, which lead to having some components automatically updated and some not. This constraint should make it a bit easier and more consistent. Also, I trust symfony to properly follow semantic versioning.

• Refactor some stuff for symfony 5:
  • transChoice -> trans
  • Routeloader
  • fos rest bundle route loader -> symfony annotations
  • use yaml instead of yml for file extensions
  • use updated web container to have website displayed again
  • Use kernel from symfony 5 demo app, rename from FoodsharingKernel
  • Use more standard symfony paths: var/ for cache/environment specific data, templates/ instead of views/ for templates

(unrelated: Remove grafana from dev setup as it didn't want to start anymore and I guess nobody is using it)

How confident are you it won't break things if deployed?

Unsure yet, we might have had some more undetected deprecations that will now fail.

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings