ci: fold the gate jobs into two per package, so the long jobs fit the 20 runners

With three gate jobs per package a code change started ~29 jobs, 22 of
them running for minutes, against the organisation's 20 concurrent jobs.
Two test shards waited three minutes for a slot and the run took 9.9
minutes instead of 8.5. Server lint and checks, and client types and
checks, now share a job each (about 4-5 minutes, still shorter than a
shard), which leaves 20 long jobs. The note at the top of test.yml says
to count them before adding one.

scripts/ci/changed-areas.mjs

@@ -49,7 +49,7 @@ export const AREAS = {
     'package-lock.json',
     '.nvmrc',
     'sonar-project.properties',
-    // lint:knip in server-checks reads it.
+    // lint:knip in server-lint-checks reads it.
     'knip.jsonc',
     'Dockerfile',
     ...READ_BY_TESTS,