Fix git access denied errors when using Spring

What does this MR do?

When allowing git disk access, we call Gitlab::GitalyClient::StorageSettings.allow_disk_access which tracks this using a class instance variable.

When classes are unloaded by Spring, we still keep a stale instance of Gitlab::GitalyClient::StorageSettings in Gitlab.config. And when self.class.disk_access_denied? is called on that instance, it was checking the class instance variable of the stale class.

We fix this by using Gitlab::GitalyClient::StorageSettings.disk_access_denied? so that both of them use the same class instance variable.

Steps to reproduce the problem (You can use any other spec)

1. bin/spring stop
2. bin/rspec spec/models/key_spec.rb:6
3. touch app/models/key.rb
4. bin/rspec spec/models/key_spec.rb:6

You will see:

An error occurred while loading ./spec/models/key_spec.rb.
Failure/Error: raise DirectPathAccessError, "git disk access denied"

Gitlab::GitalyClient::StorageSettings::DirectPathAccessError:
  git disk access denied

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