Standby fails to rejoin when associated replication slot has been removed

The Standby is not able to recover properly once the replication slot has been removed. This can happen if a standby is stopped for a while and then started back up.

```
2023-02-04T22:28:59Z app[9080563f609108] ord [info]postgres | 2023-02-04 22:28:59.448 UTC [692] STATEMENT: START_REPLICATION SLOT "repmgr_slot_1094506454" 0/8000000 TIMELINE 1
2023-02-04T22:28:59Z app[9185e0ef4712d8] ord [info]postgres | 2023-02-04 22:28:59.449 UTC [578] LOG: waiting for WAL to become available at 0/8002000
2023-02-04T22:29:04Z app[9185e0ef4712d8] ord [info]postgres | 2023-02-04 22:29:04.449 UTC [632] FATAL: could not start WAL streaming: ERROR: replication slot "repmgr_slot_1094506454" does not exist
2023-02-04T22:29:04Z app[9080563f609108] ord [info]postgres | 2023-02-04 22:29:04.449 UTC [704] ERROR: replication slot "repmgr_slot_1094506454" does not exist
2023-02-04T22:29:04Z app[9185e0ef4712d8] ord [info]postgres | 2023-02-04 22:29:04.450 UTC [578] LOG: waiting for WAL to become available at 0/8002000
2023-02-04T22:29:04Z app[9080563f609108] ord [info]postgres | 2023-02-04 22:29:04.449 UTC [704] STATEMENT: START_REPLICATION SLOT "repmgr_slot_1094506454" 0/8000000 TIMELINE 1
2023-02-04T22:29:09Z app[9185e0ef4712d8] ord [info]postgres | 2023-02-04 22:29:09.453 UTC [635] FATAL: could not start WAL streaming: ERROR: replication slot "repmgr_slot_1094506454" does not exist
2023-02-04T22:29:09Z app[9185e0ef4712d8] ord [info]postgres | 2023-02-04 22:29:09.453 UTC [578] LOG: waiting for WAL to become available at 0/8002000
2023-02-04T22:29:09Z app[9080563f609108] ord [info]postgres | 2023-02-04 22:29:09.453 UTC [713] ERROR: replication slot "repmgr_slot_1094506454" does not exist
2023-02-04T22:29:09Z app[9080563f609108] ord [info]postgres | 2023-02-04 22:29:09.453 UTC [713] STATEMENT: START_REPLICATION SLOT "repmgr_slot_1094506454" 0/8000000 TIMELINE 1
```