Fix: backwards-compatible versioned API response for custom field select fields (#8912)

docs/api.md

@@ -541,6 +541,12 @@ server, the following procedure should be performed:
 2.  Determine whether the client is compatible with this server based on
     the presence/absence of these headers and their values if present.
 
+### API Version Deprecation Policy
+
+Older API versions are guaranteed to be supported for at least one year
+after the release of a new API version. After that, support for older
+API versions may be (but is not guaranteed to be) dropped.
+
 ### API Changelog
 
 #### Version 1
@@ -573,3 +579,11 @@ Initial API version.
 #### Version 6
 
 -   Moved acknowledge tasks endpoint to be under `/api/tasks/acknowledge/`.
+
+#### Version 7
+
+-   The format of select type custom fields has changed to return the options
+    as an array of objects with `id` and `label` fields as opposed to a simple
+    list of strings. When creating or updating a custom field value of a
+    document for a select type custom field, the value should be the `id` of
+    the option whereas previously was the index of the option.