Update verbose name + help text for JIRA username and password fields

[valentijnscholten]

To accompany #12250 (but also good on itself)

EDIT1:

Because these changes are only to textual Django fields that are not affecting the database sql schema it's safe to edit the existing migration that introduced these fields. Initially I had created a migration but it would conflict with other PRs so this is more efficient.

Tested upgrade from bugfix to code with this PR included locally. It works and passes the "no db model changes check" on startup and also passes docker compose exec uwsgi bash -c "python manage.py makemigrations"

EDIT2:
Turns out the help_text and verbose_name were not shown on the JIRA forms as they got hidden because of redeclaring the password field in the form classes. I linked to the texts there as well.

[dryrunsecurity]

This pull request suggests a potential input validation concern with increasing username and password field lengths to 2000 characters, which might require additional validation to prevent potential input abuse.

💭 Unconfirmed Findings (1)

Vulnerability	Potential Input Validation Concern
Description	Increasing maximum field length to 2000 characters for username and password fields could allow overly long inputs, which may require careful input validation to prevent potential abuse

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All finding details can be found in the DryRun Security Dashboard.

[mtesauro]

Approved

[Maffooch]

@valentijnscholten I apologize for the flip flop here, but can this PR go against dev instead? There is already a 226 migration over there, so this one would complicate the release process a little bit

[valentijnscholten]

@Maffooch I made two changes, see EDIT1 and EDIT2 above.
(re-)Requested reviews because of the changes.

[Maffooch]

There has been a recent rule to not allow changes to existing migrations any longer. Can you please make new migrations, and then base this PR against the dev branch?

[github-actions]

Conflicts have been resolved. A maintainer will review the pull request shortly.