Allow user to skip archive creation on backup

What does this MR do?

Allows the user to skip creation of an archive of the generated backup by specifying SKIP=tar. This leads to a situation where only one backup can be stored on the server.

Allow restore from such a backup (as the timestamp is not (easily) available, prioritise such a backup).

Document the feature.

I've tried to be careful not to change the default behaviour of the rake task, but as I don't have access to any cloud storage I haven't been able to test the functionality to upload generated backups to such.

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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