Server-Timing: Leverage core template enhancement output buffer in WordPress 6.9+

[sarthak-19]

Solved Issue : #2225

Summary

When WordPress 6.9+ is available (Core-43258, r60936), Performance Lab no longer needs to open its own output buffer for Server-Timing metrics. Instead, it leverages the new core infrastructure:

• With output buffering enabled: Uses the wp_template_enhancement_output_buffer filter instead of starting a custom ob_start() at template_redirect.
• Without output buffering: Uses the wp_before_include_template action instead of misusing the template_include filter.

On older WordPress versions, the existing behavior is preserved as a fallback.

[westonruter]

If implement this after #2354 then we won't need to add the back-compat code to be compatible with <6.9.