multitenant: add SQL server startup guardrails

This code change prevents a SQL server from starting if its
binary version is less than the tenant's active version.

Release note: None
Epic: CRDB-10829

pkg/server/server_sql.go

@@ -1459,6 +1459,16 @@ func (s *SQLServer) preStart(
 	if err := s.settingsWatcher.Start(ctx); err != nil {
 		return errors.Wrap(err, "initializing settings")
 	}
+
+	// Prevent the server from starting if its binary version is too low
+	// for the current tenant cluster version.
+	tenantActiveVersion := s.settingsWatcher.GetTenantClusterVersion().ActiveVersion(ctx).Version
+	if s.execCfg.Settings.Version.BinaryVersion().Less(tenantActiveVersion) {
+		return errors.Newf("preventing SQL server from starting because its binary version "+
+			"is too low for the tenant active version: server binary version = %v, tenant active version = %v",
+			s.execCfg.Settings.Version.BinaryVersion(), tenantActiveVersion)
+	}
+
 	if err := s.systemConfigWatcher.Start(ctx, s.stopper); err != nil {
 		return errors.Wrap(err, "initializing settings")
 	}

pkg/server/settingswatcher/settings_watcher.go

@@ -440,3 +440,7 @@ func (s *SettingsWatcher) GetStorageClusterVersion() clusterversion.ClusterVersi
 	}
 	return s.mu.storageClusterVersion
 }
+
+func (s *SettingsWatcher) GetTenantClusterVersion() clusterversion.Handle {
+	return s.settings.Version
+}