collab: Backfill github_user_created_at on users (#16600)

This PR adds a backfiller to backfill the `github_user_created_at`
column on users.

Release Notes:

- N/A

crates/collab/k8s/collab.template.yml

@@ -216,6 +216,11 @@ spec:
                 secretKeyRef:
                   name: supermaven
                   key: api_key
+            - name: USER_BACKFILLER_GITHUB_ACCESS_TOKEN
+              valueFrom:
+                secretKeyRef:
+                  name: user-backfiller
+                  key: github_access_token
             - name: INVITE_LINK_PREFIX
               value: ${INVITE_LINK_PREFIX}
             - name: RUST_BACKTRACE