Migrate from Bitnami etcd images - no longer freely available

[mackenzie-3spread]

The APISIX Docker examples currently use Bitnami etcd images (e.g., bitnami/etcd:3.4.9), which are no longer freely available as of September 29, 2025. All versioned Bitnami images have been moved to an unmaintained legacy repository or require expensive enterprise subscriptions ($50K-$72K/year).
Impact

New users cannot deploy APISIX using current documentation
Existing deployments may fail with ErrImagePull errors
No security updates available for legacy images

Proposed Solution
Migrate to official etcd images maintained by the etcd project:

quay.io/coreos/etcd:v3.5.x (recommended)

These are the official images recommended by the etcd project and receive regular updates.
Files to Update

docker-compose.yml and variants
Documentation examples
README.md

[github-actions]

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