Use "Not Masked" as opposite of "Masked" in CI Variables UI

Summary

The current UI for CI/CD variables uses "Visible" as the opposite of "Masked", which is confusing and potentially misleading, especially for file-type variables.

Problem

As discussed in #579286, the terminology "Visible" doesn't clearly communicate that it's the opposite of "Masked". This is particularly problematic because:

1. Users need to set file variables (like SSH keys) to "Visible" to allow newlines and other special characters, but file type variables are usually not at risk of accidentally being exposed in logs.
2. Telling users to set their private SSH key to "Visible" creates confusion about security implications
3. The API uses masked: true/false, not a separate "visible" attribute

Proposal

Replace "Visible" with "Not Masked" in the CI/CD variables UI to:

• Better align with the API terminology (masked attribute)
• More clearly communicate the relationship between the two options
• Reduce confusion about security implications
• "Not Masked" is preferred over "Unmasked" (which implies it was once masked)

Related Issues

• #579286 - Original docs feedback issue about newlines in file variables