Don't distribute ClickHouse and NATS

NATS is becoming an optional GitLab core dependency sooner than
expected, which changes the delivery approach this ADR previously
proposed.

Both NATS and ClickHouse now have use cases outside of DIP, and a
customer should be able to run one instance of each, shared across
every consumer. Bundling either into a DIP package would make it
DIP-owned: its version, topology, and upgrade window set by the DIP
release. The ADR now proposes shipping neither, and instead testing
and documenting a minimal configuration for both.

With the dependencies out of the packages, DIP has a single component
left to ship, so it is delivered as the Siphon chart rather than as a
DIP bundle.