chore: replace depot.dev runners with GitHub-hosted runners (#17641)

## Summary
- Replace all `depot-ubuntu-24.04-8` runner references with the
equivalent GitHub-hosted `ubuntu-latest-8-cores` runner
- Updated across 4 workflow files: `ci-front.yaml`, `ci-server.yaml`,
`ci-emails.yaml`, `ci-sdk.yaml`
- Also updated cache key names in `ci-front.yaml` that referenced the
depot runner name

## Test plan
- [ ] Verify CI workflows run successfully on the new GitHub-hosted
larger runners
- [ ] Confirm cache keys work correctly with the updated naming

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/ci-sdk.yaml

@@ -47,7 +47,7 @@ jobs:
   # TODO uncomment when syncApplication resolver is fixed
   # sdk-e2e-integration-test:
   #   timeout-minutes: 30
-  #   runs-on: depot-ubuntu-24.04-8
+  #   runs-on: ubuntu-latest-8-cores
   #   needs: [changed-files-check, sdk-test]
   #   strategy:
   #     matrix: