Add 429 response code #19245

Description:

This PR adds a 429 response code to Common::sendResponseCode(). and wraps the Zend_Db_Table::setDefaultAdapter($adapter); in a conditional This helps prevent a reference being kept, making it easier to close a DB connection if required.

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed