Changes

chore(prow-jobs/pingcap/tidb): update TiDB periodic ut jobs to monthly schedule and reduce resources (#3968)

## Why

The jobs are unstable and no need to run so many times.

Update container image to v2025.11.30-5-gd8d742b-go1.25 Remove codecov
reporting and node affinity requirements Reduce memory and CPU limits
for both unit test jobs
 (commit: 891b2ed)