Fixes static analysis failure on Resolver

What does this MR do?

Static analysis job is failing on master https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/143726319.

This MR fixes that by adding frozen_string_literal on Gitlab::Database::LoadBalancing::Resolver

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9122

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] EE specific content should be in the top level /ee folder
• [-] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• [-] Security reports checked/validated by reviewer