Add DEADLOCK_DETECTED Enumeration (#1380)

This commit includes an additional value to represent the PostgreSQL error code for the condition "deadlock_detected".

- Include enum value for DEADLOCK_DETECTED PSQLState
- Set value to "40P01" per documentation located at: https://www.postgresql.org/docs/current/errcodes-appendix.html

pgjdbc/src/main/java/org/postgresql/util/PSQLState.java

@@ -73,6 +73,7 @@ public enum PSQLState {
 
   INVALID_SAVEPOINT_SPECIFICATION("3B000"),
 
+  DEADLOCK_DETECTED("40P01"),
   SYNTAX_ERROR("42601"),
   UNDEFINED_COLUMN("42703"),
   UNDEFINED_OBJECT("42704"),