Only `amd64` and `arm64` actually work.

The KeyDB role includes a validation task and will complain about
unsupported architectures (like `arm32`).

`arm32` users can stick to Redis for now (`keydb_enabled: false` + `redis_enabled: true`) until:
- the KeyDB role starts supporting self-building.. although building such large
  projects on weak CPUs is probably impractical
- a prebuilt arm32 image is made available by other means