Remove slim from GET /api/v2/task-group/

[ychiucco]

closes #3384

Checklist before merging into main

• I merged main into the current branch.
• I added an appropriate entry to CHANGELOG.md
• All GitHub checks completed successfully, or there are failures which are clearly unrelated to this Pull Request and I opened an issue about it.
• The reported "PR coverage" is 100%, or we briefly discussed why this is not the case.
• I added logging to new code - if appropriate.
• I added type hints to new code - if appropriate - and maybe I ran this command to spot some naive errors:

ty check $(git diff main --name-only ./fractal_server)

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
fractal_server/app/routes/api/v2
_aux_task_group_disambiguation.py
task_group.py
fractal_server/app/schemas/v2
task.py
Project Total

This report was generated by python-coverage-comment-action

[tcompa]

cc @dominique-roux this PR removes a query parameter, and the default behavior changes from including many task attributes to including only a few (notably dropping e.g. the JSON schemas, to reduce the response-body size).

Can you please double check if this would break any existing use case?