fix(server): skip Sentinel checks on unreachable servers

- CheckAndStartSentinelJob returns early when the server is not
  functional. It now reads the container state with `docker ps --filter`,
  so an SSH failure throws and is no longer treated as a missing
  container that needs a restart.
- ServerManagerJob only dispatches the Sentinel version check for
  functional servers.
- On cloud, ServerPatchCheckJob runs on the maintenance queue and
  ServerLimitCheckJob runs on the webhooks queue.
- The default number of cloud maintenance workers goes from 10 to 60.
- excludeCertainErrors drops its unused ignore list and is typed as
  `never`.
- Docs and the Horizon config comments are updated, and tests cover the
  new behavior.

tests/Unit/ServerManagerJobSentinelCheckTest.php

@@ -60,6 +60,16 @@ it('dispatches an hourly Sentinel version check for a healthy Sentinel', functio
     });
 });
 
+it('skips the hourly Sentinel version check when the server is unreachable or unusable', function (string $flag) {
+    $server = createSentinelCheckServer($this->team, Carbon::now());
+    Carbon::setTestNow(Carbon::now()->setMinute($server->id % 60));
+    $server->settings->update([$flag => false]);
+
+    (new ServerManagerJob)->handle();
+
+    Queue::assertNotPushed(CheckAndStartSentinelJob::class);
+})->with(['is_reachable', 'is_usable']);
+
 it('skips the hourly SSH version check when Sentinel reports its version on push', function () {
     $server = createSentinelCheckServer($this->team, Carbon::now());
     Cache::put(Server::sentinelReportedVersionCacheKey($server->id), '1.0.2', now()->addHours(2));