[BIP-906] Deprecation of Polygon zkEVM, Fraxtal, and Mode

## Summary This proposal aims to deprecate Balancer support on three chains: 1. Polygon zkEVM - Full shutdown; all support ends at network EOL (Q2 2026) 2. Fraxtal - Degraded support; withdrawal-only functionality via portfolio page 3. Mode - Degraded support; withdrawal-only functionality via portfolio page In line with [BIP-887](https://forum.balancer.fi/t/bip-887-transitioning-to-balancer-v3-disabling-v2-pool-factories/6874), no new Balancer v2 pools can be created on any network. This...