debugger: Remove PHP debug adapter (#34020)

This commit removes the PHP debug adapter in favor of a new version
(0.3.0) of PHP extension.
The name of a debug adapter has been changed from "PHP" to "Xdebug",
which makes this a breaking change in user-configured scenarios

Release Notes:

- debugger: PHP debug adapter is no longer shipped in core Zed editor;
it is now available in PHP extension (starting with version 0.3.0). The
adapter has been renamed from `PHP` to `Xdebug`, which might break your
user-defined debug scenarios.

crates/debugger_ui/src/tests/new_process_modal.rs

@@ -290,7 +290,6 @@ async fn test_dap_adapter_config_conversion_and_validation(cx: &mut TestAppConte
     let mut expected_adapters = vec![
         "CodeLLDB",
         "Debugpy",
-        "PHP",
         "JavaScript",
         "Delve",
         "GDB",