Drop PostgreSQL 12 support

12 and below.

Author: kovmir

pg_squeeze.c

@@ -58,8 +58,8 @@ extern PGDLLIMPORT int wal_segment_size;
 extern PGDLLIMPORT bool FirstSnapshotSet;
 #endif
 
-#if PG_VERSION_NUM < 120000
-#error "PostgreSQL version 12 or higher is required"
+#if PG_VERSION_NUM < 130000
+#error "PostgreSQL version 13 or higher is required"
 #endif
 
 #ifdef PG_MODULE_MAGIC_EXT