Ignore test and docs folders in dependency scanning

What does this MR do?

This change tweaks the dependency scanning configuration to ignore test and doc folders. In consequence, the dependency scanning report will not report vulnerable dependencies in these folders. Because such dependencies are not shipped with GitLab (i.e. do not land in our Omnibus distribution), we consider these findings to be false positives.

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

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

/cc @gitlab-com/gl-security/appsec