fix(distros): clarify misleading ssh_redirect_user warning

[Sandhya-d]

This PR rewords a misleading warning emitted when ssh_redirect_user is
configured but no cloud public SSH keys are available.

When ssh_redirect_user is set to true or default, cc_users_groups
resolves it to the actual default username before create_user runs. The
"no cloud public-keys" warning then printed that resolved name (e.g.
ubuntu), making it look like the user had configured ubuntu. This
rewords the warning so the value reads as the redirect target, not the
configured value.

Fixes GH-3924

Proposed Commit Message

```
fix(distros): clarify misleading ssh_redirect_user warning

When ssh_redirect_user is set to true or default, cc_users_groups
resolves it to the actual default username before create_user runs.
The 'no cloud public-keys' warning then printed that resolved name
(e.g. 'ubuntu'), making it look like the user had configured 'ubuntu'.
Reword the warning so the value reads as the redirect target, not the
configured value.

Fixes GH-3924
```

Additional Context

The warning is emitted in Distro.create_user
(cloudinit/distros/__init__.py). The value substitution happens earlier
in cc_users_groups, which replaces a true/default ssh_redirect_user
with the resolved default username before it reaches create_user.

Test Steps

• pytest tests/unittests/distros/test_create_users.py — 59 passed,
  including test_create_user_with_ssh_redirect_user_no_cloud_keys.
• Ran tox -e do_format, tox -e ruff, tox -e mypy, and tox -e pylint
  locally — all clean.

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[blackboxsw]

Unrelated unittest failures fixed by #6908

[blackboxsw]

Thank you for this contribution. unrelated unittest failures in CI are fixed in a separate PR.