fix(azure-aks-multicluster): move Aviatrix gateways off deprecated DSv3 family

[cmchenr]

Description

The 4 Aviatrix gateways in this blueprint's network layer (transit, frontend spoke, backend spoke, DB spoke) hardcode instance_size = "Standard_D2s_v3". Azure has deprecated the Standard DSv3 family for new self-service quota grants — a quota-increase request against standardDSv3Family is rejected outright with DeprecatedQuotaType, even when the subscription hasn't hit its current limit. On a subscription where DSv3 headroom is already consumed by other workloads (e.g. by other resources sharing the same region), this blueprint becomes undeployable with no self-service remediation path.

This PR switches all 4 gateways to Standard_D2s_v5 — same 2 vCPU / 8 GB shape, same $0.096/hr on-demand rate in eastus2 (verified via the Azure Retail Prices API at time of writing) — which is still eligible for self-service quota increases. Updated the README's quota table, fail-fast pre-flight check, resource tables, cost tables, and family-quota callout to match.

Type of Change

• New blueprint
• Blueprint enhancement
• Bug fix
• Documentation update
• CI/CD improvement
• Other (describe):

Blueprint Checklist (for new or modified blueprints)

Documentation

• README.md includes all required sections (see Blueprint Standards)
• All variables are documented with descriptions — unaffected, no new variables
• Architecture diagram is included and accurate — n/a, no architecture change
• terraform.tfvars.example includes all required variables — n/a, no new variables
• Test scenarios are documented — n/a, no new test scenario needed
• Troubleshooting/Prerequisites section covers this change (quota table + note)

Code Quality

• terraform fmt passes
• terraform validate passes
• No hardcoded values (use variables) — unaffected; this is a fix to an existing hardcoded VM size, not a new one
• Sensitive variables marked as sensitive = true — unaffected
• Resource naming uses var.name_prefix — unaffected

Testing

• Full deploy/destroy cycle tested — not re-run for this change; verified via terraform fmt/validate only (VM size swap, no new resources)
• All test scenarios verified — unaffected
• Tested on documented Control Plane version(s)
• Cleanup leaves no orphaned resources — VM size is not a forces-replacement-safe concern for cleanup; unaffected

Catalog Update

• Blueprint added to catalog in root README.md — n/a, not a new blueprint

Control Plane Version Tested

• Control Plane version: N/A for this change (config-only; validated with terraform validate against provider ~> 8.2)

Cloud Environment

• Cloud provider(s): Azure
• Region(s) tested: N/A for this change (pricing/quota verified via API for eastus2; live deploy tracked separately)

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