fix: upgrade fern-go-sdk generator to 1.28.3 to fix preview-go CI

The preview-go CI check has been failing since at least Feb 16 across
all PRs touching fern/ files. The root cause is the outdated Go SDK
generator version (1.16.4) being unable to compile code generated from
the current OpenAPI spec. Upgrading to 1.28.3 (latest) includes fixes
for datetime handling, marshal/unmarshal code generation, and other
compilation issues.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Vapi Tasker

fern/apis/api/generators.yml

@@ -51,7 +51,7 @@ groups:
   go-sdk:
     generators:
       - name: fernapi/fern-go-sdk
-        version: 1.16.4
+        version: 1.28.3
         disable-examples: true
         api:
           settings: