Fix missing setting LDAP servers

What does this MR do?

Fix for #299608 (closed)

If LDAP is disabled in Omnibus, the LDAP servers configuration hash is not present. GitLab relies on this hash being present even when LDAP is disabled. Now we initialize the hash on startup.

This bug occurred due to a recent change in !51132 (merged). At that time it wasn't known that the hash syntax for SettingsLogic meant 'optional'. Regardless, the hash notation made the setting very difficult to stub in tests. Rather than go back and try to fix all the tests, it's simple and reasonable to initialize the servers hash on startup. We do this with lots of settings and it seems like the most minimal step to solve the problem.

Settingslogic::MissingSetting (Missing setting 'servers' in 'ldap' section in /opt/gitlab/embedded/service/gitlab-rails/config/gitlab.yml):
  
lib/gitlab/auth/ldap/config.rb:31:in `servers'
lib/gitlab/auth/ldap/config.rb:45:in `providers'
lib/gitlab/auth/o_auth/user.rb:153:in `ldap_person'
lib/gitlab/auth/o_auth/user.rb:146:in `creating_linked_ldap_user?'
lib/gitlab/auth/o_auth/user.rb:244:in `update_profile'
lib/gitlab/auth/o_auth/user.rb:29:in `initialize'

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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• Style guides
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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

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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