fix: stop mutating auth.User help_text to silence phantom migration warning

[CryptoRodeo]

admin.py set help_text on Django's built-in User.username field at import time. Django tracks help_text in migrations, so every deploy warned that the auth app was out of sync but auth is Django-owned, so it could never be fixed.

Dropped the line. The help text is already set on the admin form, so nothing changes for users. Kept the nearby validators override, it doesn't affect migrations.

Verified in the dev container: migrate now runs clean.

JIRA: PULP-2263

Summary by Sourcery

Bug Fixes:

• Prevent recurring Django auth migration warnings by removing the import-time mutation of the built-in User username field’s tracked help text.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Stops mutating Django’s built‑in auth.User.username help_text at import time to eliminate spurious migration warnings, while retaining the custom username validator override and documenting the migration-tracking behavior in comments.

File-Level Changes

Change	Details	Files
Stop overriding auth.User.username help_text in admin customization to avoid phantom migration warnings while keeping the custom validator override.	Left the RegexValidator-based username validator and assignment to User._meta.get_field('username').validators in place. Removed the assignment that set User._meta.get_field('username').help_text to a custom USERNAME_HELP_TEXT value. Expanded the surrounding comment to explain which field attributes are safe to mutate (validators) and which are tracked by Django’s migration autodetector (help_text), and where the user-facing help text is actually set.	pulp_service/pulp_service/app/admin.py
Document the bugfix in the changelog system for auth migration warnings.	Added a changelog entry file describing the fix for the auth migration warning caused by help_text mutation.	CHANGES/+auth-migration-warning.bugfix

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