chore: fix PR template links to execution-specs

[danceratopz]

🗒️ Description

• Fix hardcoded links pointing to execution-spec-tests instead of execution-specs, and use HEAD instead of main for branch references.
• Removed CHANGELOG link, for now, follow-up issue: chore: add per-package changelog files #2036

🔗 Related Issues or PRs

N/A.

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks
• All: PR title adheres to the repo standard
• All: Considered updating the online docs in the ./docs/ directory.
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.
• Tests: For PRs implementing a missed test case, update the post-mortem document to add an entry the list.
• Ported Tests: All converted JSON/YML tests from ethereum/tests or tests/static have been assigned @ported_from marker.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.07%. Comparing base (05117e5) to head (ada35db).
⚠️ Report is 2 commits behind head on forks/amsterdam.

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #2035   +/-   ##
================================================
  Coverage            86.07%   86.07%           
================================================
  Files                  599      599           
  Lines                39527    39527           
  Branches              3780     3780           
================================================
  Hits                 34021    34021           
  Misses                4872     4872           
  Partials               634      634           

Flag	Coverage Δ
unittests	86.07% <ø> (ø)

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[spencer-tb]

LGTM! Bye bye Changelog 👋 🪦