Fix default-branch pipeline cache action

[ohmyfelix]

Summary

• replace all actions/cache@v2 usages in .github/workflows/main.yaml with actions/cache@v4
• resolve the current default-branch workflow hard failure caused by GitHub disabling actions/cache@v2
• keep the workflow structure unchanged to minimize risk

Verification

• composer validate
• inspected latest failing run log (gh run view 21818641681 --log) to confirm root cause before patching