chore: update eredis to 1.2.18

[hjianbo]

Summary

• update eredis dependency from 1.2.17 to 1.2.18
• bump eredis_cluster version metadata from 0.8.11 to 0.8.12

Why

eredis 1.2.18 separates Redis data-node credentials from Redis Sentinel credentials, so eredis_cluster needs to consume this driver version before EMQX can update its eredis_cluster dependency.

Checks

• rebar3 compile
• rebar3 xref
• docker compose up -d --wait
• rebar3 eunit -v -c
• rebar3 cover -v
• git diff --check
• docker compose down