fix(code-mapping): Fix trace information for code_mappings (#46018)

Use `start_span` to capture more fine-tuned details of the performance
for the derive code mappings task.

WOR-2329

src/sentry/integrations/github/client.py

@@ -361,7 +361,7 @@ class GitHubClientMixin(ApiClient):  # type: ignore
         if page_number_limit is None or page_number_limit > self.page_number_limit:
             page_number_limit = self.page_number_limit
 
-        with sentry_sdk.start_transaction(
+        with sentry_sdk.start_span(
             op=f"{self.integration_type}.http.pagination",
             name=f"{self.integration_type}.http_response.pagination.{self.name}",
             parent_span_id=parent_span_id,