Rename initialize package to cacert

What does this MR do?

Rename the initialize Go package introduced in !76 (merged) and align it with the other packages of the common library. Overall things are changed to make it clear we're dealing with CA certificates.

cacert.NewBundle returns an object that combines a CA bundle with its import path. It's imported by calling the Import() method. Warning! This method is a no-op if the CA bundle is empty, so that callers don't have to deal with nil objects; it's optimized for ease of use.

What are the relevant issue numbers?

• SAST: gitlab-org/gitlab#11797 (closed)
• Dependency Scanning: gitlab-org/gitlab#208721 (closed)
• Container Scanning: gitlab-org/gitlab#208720 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• [-] Documentation created/updated for GitLab EE, if necessary
• [-] Documentation created/updated for this project, if necessary
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Job definition updated, if necessary
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer