Do not update Repository attributes on delete, add docs

[awcrosby]

Description 🛠

Stop updating Repository.retain_repo_versions on delete task, so that if
retain_repo_versions has been manually set, the system will not
override.

Update tasks/deletion.py module docs to describe Content removal vs.
deletion and how retain_repo_versions is related.

No-Issue

Reviewer Checklists 👀

Developer reviewer:

• Code looks sound, good architectural decisions, no code smells
• There is a Jira issue associated (note that "No-Issue" should be rarely used)
• Tests are included in galaxy_ng/tests/integration or galaxy_ng/tests/functional, and they fully cover necessary test scenarios… or tests not needed

QE reviewer (exceptions):

• Tests are included in galaxy_ng/tests/integration or galaxy_ng/tests/functional, and they fully cover necessary test scenarios… or tests not needed
• PR meets applicable Acceptance Criteria for associated Jira issue

Note: when merging, include the Jira issue link in the squashed commit