pghoard doesn't like rc versions

```
File "/usr/bin/pghoard", line 33, in
sys.exit(load_entry_point('pghoard==2.2.1.dev32', 'console_scripts', 'pghoard')())
File "/usr/lib/python3.9/site-packages/pghoard/pghoard.py", line 896, in main
pghoard = PGHoard(config_path)
File "/usr/lib/python3.9/site-packages/pghoard/pghoard.py", line 84, in __init__
self.load_config()
File "/usr/lib/python3.9/site-packages/pghoard/pghoard.py", line 789, in load_config
new_config = config.read_json_config_file(self.config_path)
File "/usr/lib/python3.9/site-packages/pghoard/config.py", line 179, in read_json_config_file
return set_and_check_config_defaults(config, check_commands=check_commands, check_pgdata=check_pgdata)
File "/usr/lib/python3.9/site-packages/pghoard/config.py", line 156, in set_and_check_config_defaults
site_config[command + "_version"] = convert_pg_command_version_to_number(version_string)
File "/usr/lib/python3.9/site-packages/pghoard/common.py", line 162, in convert_pg_command_version_to_number
raise Error("Unrecognized PostgreSQL version string {!r}".format(command_version_string))
pghoard.rohmu.errors.Error: Unrecognized PostgreSQL version string 'pg_basebackup (PostgreSQL) 14rc1'
```