fix: move proxy-port properties to proxy plugin and fix CI failures

- Move proxy-port and proxy-ssl-port properties from ports plugin to
  proxy plugin where they conceptually belong
- Generalize proxy:set to handle any property (matching ps:set pattern)
  instead of only handling the type property
- Fix apps:set --global flag parsing (pflag treats --global as a flag,
  not an argument — must be defined explicitly like ps:set does)
- Add missing property-functions source to scheduler-docker-local
  shell scripts (scheduler-enter, scheduler-run, scheduler-deploy,
  check-deploy, bin/scheduler-deploy-process-container)
- Update nginx-vhosts property references from ports to proxy plugin
- Update tests to use proxy:set for proxy-port properties

docs/configuration/environment-variables.md

@@ -122,8 +122,8 @@ The following environment variables have been migrated to plugin properties. Exi
 | `DOKKU_DISABLE_PROXY` | `dokku proxy:disable <app>` / `dokku proxy:enable <app>` |
 | `DOKKU_DOCKERFILE_START_CMD` | `dokku ps:set <app> dockerfile-start-cmd <value>` |
 | `DOKKU_PARALLEL_ARGUMENTS` | Removed. No longer supported. |
-| `DOKKU_PROXY_PORT` | `dokku ports:set <app> proxy-port <value>` |
-| `DOKKU_PROXY_SSL_PORT` | `dokku ports:set <app> proxy-ssl-port <value>` |
+| `DOKKU_PROXY_PORT` | `dokku proxy:set <app> proxy-port <value>` |
+| `DOKKU_PROXY_SSL_PORT` | `dokku proxy:set <app> proxy-ssl-port <value>` |
 | `DOKKU_SKIP_ALL_CHECKS` | `dokku checks:disable <app>` |
 | `DOKKU_SKIP_CLEANUP` | `dokku builder:set <app> skip-cleanup <true\|false>` |
 | `DOKKU_SKIP_DEFAULT_CHECKS` | `dokku checks:skip <app>` |