securedrop-admin integration tests fail during "tag is live, release is not" time window

[eloquence]

Discovered during 1.8.0 release (#5794): If a tag has been pushed but the release object has not been created yet, two securedrop-admin tests start failing:

• test_check_for_update_when_updates_not_needed
• test_update_with_duplicate_branch_and_tag

Those tests operate against the production repo, and they rely on the latest release and the latest tag being identical. We should document this behavior in the RM guide, or make the tests more resilient. See #5862 (comment) for test output.

(We confirmed that this was indeed the cause by re-running CI after the release object was created.)

[eloquence]

Discussed with @emkll at backlog review, we'll probably just document this behavior for now.