Fallback to lowest visibility level in snippet visibility radio

What does this MR do?

In snippets form when the default visibility level is not available fallback to the lowest available visibility level.

How to reproduce

1. In admin panel select as both default and restricted a snippet visibility level

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2. Impersonate a non-admin
3. Create a new Snippet or Project snippets

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4. The lowest available visibility level will be selected

Before the fix no visibility level was selected thus a form validation error will be shown on submit.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #216871 (closed)