refactor: move code for fetching an available port to the ports plugin

Also translate the code to golang.

docs/appendices/0.31.0-migration-guide.md

@@ -8,6 +8,7 @@
 - Users no longer need to clear the `source-image` git property when transitioning from image-based deploys (`git:from-image` and `git:load-image`) to other deployment methods (git push, `git:from-archive`, `git:sync`).
 - For deploys via the `git:from-image` and `git:load-image` commands, the `CHECKS` file is now extracted from the configured `WORKDIR` property of the image. For all other deploys - git push, `git:from-archive`, `git:sync` - will have the `CHECKS` extracted directly from the source code. The filename in both cases is `CHECKS` and cannot be modified.
 - The `common#get_app_raw_tcp_ports()` function has been deprecated and will be removed in the next release. Users should avoid interacting with this function for dockerfile ports and instead use the `ports-get` plugin trigger for fetching ports for an app.
+- The `common#get_available_port()` function has been deprecated and will be removed in the next release. Users should avoid interacting with this function and instead use the `ports-get-available` plugin trigger for fetching an available port.
 
 ## Deprecations
 

docs/development/plugin-triggers.md

@@ -1288,6 +1288,21 @@ set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 # TODO
 ```
 
+### `ports-get-available`
+
+- Description: Prints out an available port greater than 1024
+- Invoked by: Various networking plugins
+- Arguments `$APP`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+# TODO
+```
+
 ### `post-app-clone`
 
 - Description: Allows you to run commands after an app was cloned.