Default masked to false for new variables

What does this MR do?

Change the default value of the masked parameter for CI Variables to false

When we introduced the masked variables feature, we set the default to true in order to encourage its use, but it has caused more confusion than good since the usable character set is so small (a limitation of the MVC to ensure that masking wouldn't cause unintended side effects). This merge request reverses that decision.

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

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/11204