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

docs/plugin-template/plugin.toml

@@ -8,7 +8,7 @@ license = "GPL-3.0-only"
 repository = "your-account/your-plugin-repo"
 
 [opencad]
-api_version = 4
+api_version = 5
 rustc_version = "__RUSTC_VERSION__"
 ribbon_order = 60
 command_prefixes = ["MP_"]