Add upgrade workflow

[jeremyjams]

• Triggered job manually (required until workflow is available on main default branch) with:
  gh workflow run .github/workflows/upgrade.yml --ref feature/upgrade-workflow -f network=hardhat:
• Related run: https://github.com/iExecBlockchainComputing/iexec-voucher-contracts/actions/runs/12832708706 ✔️
• Resources:
  • https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#workflow_dispatch
  • https://github.com/orgs/community/discussions/25219#discussioncomment-10902997
  • Running a GitHub Actions workflow that doesn't exist yet on the default branch

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.30%. Comparing base (d64f1b1) to head (543cb80).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #64   +/-   ##
========================================
  Coverage    99.30%   99.30%           
========================================
  Files            4        4           
  Lines          287      287           
  Branches        53       53           
========================================
  Hits           285      285           
  Misses           2        2           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.