fix: address release-retired-learner-email PR review feedback

[rahulkanneri-2u]

Description

Follow-up to #460 addressing the review comments that were
still open when that PR merged into release-ulmo. No new behavior is introduced release_retired_learner_email()
itself is unchanged. This is cleanup of the surrounding code and tests, plus one CLI interface change:

• Removed --username from the release_retired_user_email management command. Per review feedback, a
  tool built for retired learners shouldn't accept a username (PII-shaped input) at all --user_id is now the
  only supported identifier. This is a breaking change for anyone already scripting the old --username flag.
• Dropped the lint-amnesty tag on the User import in accounts/utils.py and in the management command —
  it's slated for removal from openedx altogether, so it shouldn't be copied into new files.
• Test setup emails now use settings.RETIRED_EMAIL_DOMAIN instead of a hardcoded retired.invalid, matching
  the assertions in the same tests and removing the inconsistency flagged in review.
• Switched the command's two "success message" tests from asserting against captured stdout (capsys) to
  mocking the logger (mock_logger.info.assert_called_with(...)), matching the existing pattern in
  common/djangoapps/student/management/tests/test_change_enrollment.py.

Supporting information

• LP-1177 (private)
• Original PR: #460

Testing instructions

1. Retire a test user to the COMPLETE state.
2. is_email_retired("learner@example.com") => True.
3. ./manage.py lms release_retired_user_email --user_id <retired_user_id>
4. is_email_retired("learner@example.com") => False; confirm re-registration succeeds.
5. Confirm the old --username <retired_username> form now fails with an "unrecognized arguments" error.
6. pytest openedx/core/djangoapps/user_api/accounts/tests/test_utils.py openedx/core/djangoapps/user_api/management/tests/test_release_retired_user_email.py

Deadline

None.

Other information

• Depends on / follows up: fix: add command to release a retired learner's email for reuse #460, already merged into release-ulmo.
• Breaking change: any existing script or runbook using release_retired_user_email --username <...> must
  switch to --user_id <...>.
• No database migration involved.