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-emails.yaml

@@ -25,7 +25,7 @@ jobs:
     needs: changed-files-check
     if: needs.changed-files-check.outputs.any_changed == 'true'
     timeout-minutes: 10
-    runs-on: depot-ubuntu-24.04-8
+    runs-on: ubuntu-latest-8-cores
     steps:
       - name: Fetch custom Github Actions and base branch history
         uses: actions/checkout@v4