Use fully qualified modules in connection proxy

What does this MR do?

An instance of this class is stored in memory in a class instance variable. When this class is stale, it causes code-reloading issues when Gitlab::Database::LoadBalancing::ConnectionProxy.const_missing is triggered.

This is a workaround so that Object.const_missing is triggered instead.

This is similar to !31512 (merged)

Sample stacktrace:

Puma caught this error: A copy of Gitlab::Database::LoadBalancing::ConnectionProxy has been removed from the module tree but is still active! (ArgumentError)
/usr/local/Cellar/asdf/0.7.6/installs/ruby/2.6.5/lib/ruby/gems/2.6.0/gems/activesupport-6.0.2.2/lib/active_support/dependencies.rb:530:in `load_missing_constant'
/usr/local/Cellar/asdf/0.7.6/installs/ruby/2.6.5/lib/ruby/gems/2.6.0/gems/bootsnap-1.4.6/lib/bootsnap/load_path_cache/core_ext/active_support.rb:61:in `block in load_missing_constant'
/usr/local/Cellar/asdf/0.7.6/installs/ruby/2.6.5/lib/ruby/gems/2.6.0/gems/bootsnap-1.4.6/lib/bootsnap/load_path_cache/core_ext/active_support.rb:17:in `allow_bootsnap_retry'
/usr/local/Cellar/asdf/0.7.6/installs/ruby/2.6.5/lib/ruby/gems/2.6.0/gems/bootsnap-1.4.6/lib/bootsnap/load_path_cache/core_ext/active_support.rb:60:in `load_missing_constant'
/usr/local/Cellar/asdf/0.7.6/installs/ruby/2.6.5/lib/ruby/gems/2.6.0/gems/activesupport-6.0.2.2/lib/active_support/dependencies.rb:214:in `const_missing'
/Users/engwan/Projects/gdk/gitlab/ee/lib/gitlab/database/load_balancing/connection_proxy.rb:67:in `read_using_load_balancer'
/Users/engwan/Projects/gdk/gitlab/ee/lib/gitlab/database/load_balancing/connection_proxy.rb:42:in `select_all'

Screenshots

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