ci: make the Notary PostgreSQL version matrix the single required conformance gate

[jeremi]

Use Case

Notary PostgreSQL conformance currently runs PostgreSQL 16, 17, and 18 both sequentially inside the Rust workspace job and in the dedicated parallel version matrix. This duplicates runtime and leaves two workflow locations responsible for the supported-major list.

The sequential gate must remain until the dedicated matrix is a stable required branch-protection context.

Proposed Behavior

• Ensure the dedicated Notary PostgreSQL workflow runs for every path that can affect the state plane or its conformance harness.
• Give the required matrix result a stable status context and make it a required merge gate.
• After branch protection is updated and verified, remove the sequential PostgreSQL conformance invocation from the Rust workspace job.
• Retain coverage for PostgreSQL 16, 17, and 18, including the supported upgrade and restore probes.

Boundaries

• No reduction in supported-major, migration, upgrade, backup/restore, readiness, or negative-path coverage.
• Do not remove the existing required invocation before the replacement requirement is active.
• Keep the supported-major list owned in one workflow location.