Use gitaly-backup for repository backups by default

What does this MR do?

#333034 (closed)

• Swaps out the ruby implementation of repository backups that calls gitaly RPCs (GitalyRPCBackup) with an executable maintained by gitaly called gitaly-backup (GitalyBackup).
• backup.rake is only used for on-prem. So we cannot use feature rollouts on gitlab.com. Instead we're enabling it by default.

This can be disabled in the rails console:

Feature.disable :gitaly_backup

Testing (GDK)

1. Make sure GDK is up-to-date and reconfigured. There's a new configuration for gitlab.yml to locate gitaly-backup that GDK needs to set up.

   $ gdk update
   $ gdk reconfigure

2. Checkout this branch

   $ git checkout enable_gitaly_backup

3. Make sure the feature is removed (rails console):

   Feature.remove :gitaly_backup

4. Create a backup (shell):

   $ bundle exec rake gitlab:backup:create
   2021-07-23 13:40:29 +1200 -- Dumping database ... 
   Dumping PostgreSQL database gitlabhq_development ... [DONE]
   2021-07-23 13:40:32 +1200 -- done
   2021-07-23 13:40:32 +1200 -- Dumping repositories ...
   time="2021-07-23T13:40:32+12:00" level=info msg="started backup" gl_project_path=gitlab-org.wiki relative_path=@groups/78/5f/785f3ec7eb32f30b90cd0fcf3657d388b5ff4297f2f9716ff66e9b69c05ddd09.wiki.git storage_name=default
   time="2021-07-23T13:40:32+12:00" level=warning msg="skipped backup" error="repository skipped" gl_project_path=gitlab-org.wiki relative_path=@groups/78/5f/785f3ec7eb32f30b90cd0fcf3657d388b5ff4297f2f9716ff66e9b69c05ddd09.wiki.git storage_name=default
   ...

If the repository backups start, this is enough to know the feature flag is working correctly.

Does this MR meet the acceptance criteria?

Conformity

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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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