Pass log level explicitly when building Gitlab::Logger

What does this MR do?

Note, the log level of Ruby's Logger defaults to DEBUG https://ruby-doc.org/stdlib-2.6.6/libdoc/logger/rdoc/Logger.html so we are not changing the log level but making it extra clear that the choice of the log level is intentional and not configurable for structured logging (yet).

Refs #247092 (comment 410387157)

Verification

Gitlab::Kubernetes::Logger.build.level == Logger::DEBUG # => true

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