fix: remove rollback_to_latest_ready_revision (incompatible with Single revision mode)

## Context
The \
ollback_to_latest_ready_revision\ function in \deploy-backend-services\ uses \z containerapp ingress traffic set --revision-weight\ to redirect traffic. This only works in **Multi** revision mode.

## Problem (AzureContainerAppsSpecialist finding)
With \
evision_mode = Single\, traffic always goes to the latest revision. The traffic-weight rollback is a no-op or fails silently. This is a **latent bug** — it doesn't break anything today but provides false safety.

## Recommendation
- Remove \
ollback_to_latest_ready_revision\ function
- Let the retry loop re-issue \z containerapp update\ on failure
- OR: switch to Multi revision mode for zero-downtime deploys (bigger change)

## Files
- \.github/workflows/azd-deploy.yml\ — \deploy-backend-services\ job