replication_applier_status_by_worker requires mysql 8.0 (#683)

The query in the replication_applier_status_by_worker check assumes the
replication_applier_status_by_worker table layout in mysql 8.0.

Co-authored-by: Alexander J. Maidak <alex.maidak@shopify.com>

collector/perf_schema_replication_applier_status_by_worker.go

@@ -96,7 +96,7 @@ func (ScrapePerfReplicationApplierStatsByWorker) Help() string {
 
 // Version of MySQL from which scraper is available.
 func (ScrapePerfReplicationApplierStatsByWorker) Version() float64 {
-	return 5.7
+	return 8.0
 }
 
 // Scrape collects data from database connection and sends it over channel as prometheus metric.