RavenDB-24866: set user to 'ravendb' in service settings template

Author: TheGoldenPlatypus

roles/ravendb_node/templates/ravendb.service.j2

@@ -9,7 +9,7 @@ LimitNOFILE=65535
 LimitRSS=infinity
 LimitAS=infinity
 TasksMax=infinity
-User=RAVENDB_USERNAME
+User=ravendb
 StartLimitBurst=0
 Restart=on-failure
 Type=exec