pg_checksums: Use new routine to retrieve data of PG_VERSION

pg_createsubscriber: Use new routine to retrieve data of PG_VERSION

pg_resetwal: Use new routine to retrieve data of PG_VERSION

pg_combinebackup: Use new routine to retrieve data of PG_VERSION

pg_upgrade: Use new routine to retrieve data of PG_VERSION

Fix version number calculation for data folder flush in pg_combinebackup