feat(files_external/s3): make some auth labels clearer

[joshtrichards]

• Resolves: #

Summary

Changes

Improve the S3 external storage UI by making two labels more explicit:

• rename the S3 auth method label from "Access key" to a clearer static-credentials label and update the parameter names
• rename "Legacy (v2) authentication" to explicitly refer to legacy S3 signature v2 and avoid confusion with "authentication" methods (and prefix with "Use..." for consistent with other elements).

Why

The current S3 wording is confusing in two ways:

1. "Access key" is too vague
   It does not clearly communicate that this option expects long-lived static credentials, and it is easily confused with the field label itself.

2. "Legacy (v2) authentication" is ambiguous
   This setting is really about legacy S3 request signing compatibility, not the general authentication method.

These changes make the S3 configuration UI more self-explanatory without changing behavior.

Future follow-up

There is a related UX issue where "None" is misleading for S3. I was also hoping to change that (to something like "Instance metadata"). However, that is being left for a follow-up PR because it has additional design/compatibility hurdles:

• SCHEME_NULL is generic across files_external
• we re-used null::null rather than creating an S3 specific one
• Backend::addAuthScheme() works at the scheme level, not per-backend label override
• introducing an S3-specific replacement likely needs either framework changes or careful compatibility (migration) handling for existing config

For now that one will have to remain labeled "None" as-is in the UI unfortunately...

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI