Use requested version for triggered build response (#12571)

## Summary
- serialize the triggered build response using the requested version
instead of the build's version

## Fixes
https://read-the-docs.sentry.io/issues/6296397769/


------
[Codex
Task](https://chatgpt.com/codex/tasks/task_e_69148d12c8a08320b6a4ac8267d089b4)

Author: ericholscher

readthedocs/api/v3/views.py

@@ -433,7 +433,7 @@ def create(self, request, **kwargs):  # pylint: disable=arguments-differ
         data = {
             "build": BuildSerializer(build).data,
             "project": ProjectSerializer(project).data,
-            "version": VersionSerializer(build.version).data,
+            "version": VersionSerializer(version).data,
         }
 
         if build: