Allow masking if 8 or more characters in base64

What does this MR do?

Change regular expression to allow for all characters in the base64 alphabet (https://tools.ietf.org/html/rfc4648) to be properly masked, if the string is at least 8 char long. This should allow gitlab-runner to mask $SSH_PRIVATE_KEY if any user is following the guide here: https://docs.gitlab.com/ee/ci/ssh_keys/#ssh-keys-when-using-the-docker-executor.

Related #60790 (closed)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60790

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