doc: skip v2.9.0 Docker release for consistency with official images

[vkartk]

Since v2.9.0 was skipped in the official Caddy Docker images due to breaking changes, we're maintaining consistency by also skipping v2.9.0 in caddy-cloudflare images.

The decision to skip v2.9.0 was documented in docker-library/official-images/pull/18209, where francislavoie stated:

We skipped the docker release for 2.9.0 because we noticed some breaking changes soon after release.The breaking changes were addressed in v2.9.1

• Original release notes: https://github.com/caddyserver/caddy/releases/tag/v2.9.1
• Official Docker images discussion:
  • Caddy v2.9.1 caddyserver/caddy-docker#386
  • Update to caddy v2.9.0 stable caddyserver/caddy-docker#384
• Official images PR ( Decision thread ): caddy: v2.9.1 release docker-library/official-images#18209

This issue documents why v2.9.0 was omitted from our release schedule.