Geo: Tidy up Gitlab::Geo::HealthCheck

What does this MR do?

This MR tidies up Gitlab::Geo::HealthCheck:

• Convert all to instance methods
• Remove redundant 'The ' from 'The Geo'
• Break out replication lag query into method
• Increase test coverage

What are the relevant issue numbers?

#5933 (closed)

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated via this MR
• [ ] Documentation reviewed by technical writer or follow-up review issue created
• 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