replace `T` with `_` in the backup_id

[dulhaver]

backups in barman are identified by a combination of the pg-server name and a timestamp (i.e. barman list-backup [pg-server]). The logic is completely fine but it is very hard to identify a particular backupID due to the format yyyymmddThhmmss. I guess the T stands for TIME or TIMESTAMP of something along that line but I would suggest to replace it with an underscore so it gets readable somewhat to the eye (I am aware that scripts are fine with the status quo).

server_name 20211225_123455 instead of server_name 20211225T123455

server_name 20211225_123455 is somewhat identifiable for the eye where 20211225T123455 is just gibberish (to the eye).

[mikewallace1979]

The backup ID is based on the ISO 8601 basic format and that's where the T comes from - that being the case we'd rather keep them as they are.