Be explicit about setting :masked

What does this MR do?

Currently API defaults :masked to false and browser defaults :masked to true - resolve this ambiguity by requiring QA to be explicit about which value it requires for :masked.

See https://gitlab.com/gitlab-org/quality/nightly/issues/81

QA

set masked to false via API ✅

set masked to false via browser UI ✅

set masked to true via browser UI ✅

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance 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

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