fix(plugins): gate on_load behind API v5

BREAKING CHANGE: plugins using on_load must target API v5 and declare api_version = 5.

Refs #901

crates/ocs_plugin_api/build.rs

@@ -404,7 +404,7 @@ fn generate_version_info(out_dir: &Path) {
         "acadrust_version": acadrust.version.to_string(),
         "acadrust_source": acadrust.source.as_ref().map(|s| s.to_string()),
         "rustc_version": rustc_version,
-        "api_version": 4,
+        "api_version": 5,
         "api_version_min_supported": 2,
         "build_timestamp": build_timestamp,
     });