Update copyright for 2021

Author: NikolayS

LICENSE

@@ -630,7 +630,7 @@ state the exclusion of warranty; and each file should have at least
 the "copyright" line and a pointer to where the full notice is found.
 
     postgres-checkup – Automated PostgreSQL health checks
-    Copyright © 2018-2020, Postgres.ai (https://postgres.ai), Nikolay Samokhvalov
+    Copyright © 2018-2021, Postgres.ai (https://postgres.ai), Nikolay Samokhvalov
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU Affero General Public License as published