Docs: align PR template (DART 6.16)

[jslee02]

Summary

• Backport the updated PR template to release-6.16.

Motivation / Problem

• Keep PR metadata and review expectations consistent across branches.

Changes / Key Changes

• Update .github/PULL_REQUEST_TEMPLATE.md to match the main-branch template.

Testing

• Not run (docs/metadata-only changes).

Breaking Changes

• None

Related Issues / PRs (backports)

• Backport of Docs: align PR template and contribution guidance #2451

[jslee02]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.81%. Comparing base (6586daf) to head (26c045a).
⚠️ Report is 1 commits behind head on release-6.16.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-6.16    #2452   +/-   ##
=============================================
  Coverage         58.80%   58.81%           
=============================================
  Files               385      385           
  Lines             32199    32199           
  Branches           3887     3887           
=============================================
+ Hits              18936    18937    +1     
+ Misses            13263    13262    -1     

Flag	Coverage Δ
unittests	58.81% <ø> (+<0.01%)	⬆️

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see 1 file with indirect coverage changes

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