Leftover from api-platform 3.0 upgrade (#3176)

[usu]

Summary bucket of leftovers from PR #3176

• Review changes done in https://github.com/api-platform/api-platform and take over what makes sense chore: sync relevant changes from api-platform template #3255 chore: sync changes from api-platform template #3304
• Review if we want to support multiple ApiResources in our various extensions (UriTemplateFactory, etc.)
• Plural naming for /reset_password endpoint
• Test coverage for data providers
• Invitation endpoint: align more closely to REST principle
  • remove /find from GET operation
  • evaluate DELETE instead of PATCH (not sure this is correct, though)
• re-evaluate metadata cache and check if additional configuration in services.yaml could be safely removed
  (related to api_platform.metadata.property.identifier_metadata_factory is not cached api-platform/core#5285 and Metadata cache uses ArrayAdapter instead of APCu api-platform/core#4975)