Simplify up.go concurrent setup — reduce graph/legacy branching

## Context
`up_graph.go` currently has branching logic to handle both the new exegraph path (default) and the legacy sequential workflow runner path. This transitional complexity exists because custom `workflows.up` definitions still use the old runner.

## Proposal
Once #7909 (exegraph workflow processing) is complete, remove the branching and simplify `up.go` to a single graph-based execution path.

## Dependencies
- Blocked by #7909

## References
- PR #7776 review thread: https://github.com/Azure/azure-dev/pull/7776#discussion_r3126009950
- Reviewer: @weikanglim