refactor: deprecate git_deploy_branch in favor of plugn trigger

docs/appendices/0.21.0-migration-guide.md

@@ -0,0 +1,5 @@
+# 0.21.0 Migration Guide
+
+## Deprecations
+
+- `git#git_deploy_branch()` is deprecated in favor of `plugn trigger git-deploy-branch`.

docs/development/plugin-triggers.md

@@ -531,6 +531,21 @@ verify_app_name "$APP"
 # TODO
 ```
 
+### `git-deploy-branch`
+
+- Description: Outputs the deploy branch for an app, inherited or not
+- Invoked by:
+- Arguments: `$APP`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+# TODO
+```
+
 ### `git-post-pull`
 
 - Description: