Bump minimum required WP version to 6.4

[mukeshpanchal27]

For context on why I'm opening this issue, please see https://make.wordpress.org/performance/handbook/performance-lab/version-support-policy/. Specifically, the following:

"The Performance Lab plugin commits to supporting the latest stable version of WordPress core."

Since we use some WP 6.4 functions see #1048 (comment), I propose changing the minimum WP version set to 6.4. This way, we won't need to handle safeguard or old version compatibility code.

[westonruter]

As part of this, we can eliminate any wrappers for wp_trigger_error() since it was introduced in 6.4.

[felixarntz]

+1 to bumping the minimum requirement. Let's do it for 3.0, which is a good time to do it anyway.