[v3-1-test] Fix unreachable branch and broken regex in PR-of-the-month script (#62438)

- Reorder elif chain in quick_score_prs so body_len < 20 (0.4x)
  is checked before body_len < 1000 (0.8x) — previously the < 20
  branch was unreachable since < 20 is a subset of < 1000
- Fix linked-issue regex: lookbehind matched "elated: #" instead
  of "related: #", and required exactly 5 digits — switch to a
  capture group that handles any issue number length
(cherry picked from commit 6cef0a27a000ae71273214aeb9f97aa1549d04de)

Co-authored-by: Kaxil Naik <kaxilnaik@gmail.com>