docs(proposals): add proposal to replace Redis with Valkey

[chlins]

Issue: goharbor/harbor#22935

This pull request introduces a proposal to replace Harbor's internal Redis runtime with Valkey in a carefully phased approach. The initial phase focuses on a minimal, low-risk swap of the runtime package and image name, ensuring full backward compatibility for users and existing configurations. More extensive terminology and SDK changes are explicitly deferred to a future phase.

Key highlights of the proposal:

Phase 1: Minimal Valkey Adoption

• Swap the internal Redis runtime in Harbor's bundled image with Valkey by changing the installed package from redis to valkey, and rename the image from goharbor/redis-photon to goharbor/valkey-photon. All other build directories, service names, configuration keys, environment variables, and on-disk layouts remain unchanged for maximum compatibility.
• Update deployment templates (docker-compose, offline installer, and harbor-helm) to reference the new valkey-photon image, while keeping all service names, configuration paths, and environment variables the same.
• Ensure compatibility with existing configurations, external Redis/Valkey deployments, and in-place upgrades from previous Harbor releases.

Phase 2: Future Work (Not in This Release)

• Plan for renaming internal build directories, configuration keys, and service names from redis to backend-neutral or valkey

[chlins]

@goharbor/all-maintainers Please take a look for this proposal, thanks!

[Vad1mo]

We are running our Saas registries with valkey. It was a drop in replacement

[wy65701436]

lgtm, thanks.