Mark Postgres v15 as supported for borg backup

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2257

Fixed in d134cd7c4c
(thanks to `alpine:latest` now being `alpine:3.17.0`, which includes
Postgres v15)

roles/custom/matrix-backup-borg/defaults/main.yml

@@ -37,7 +37,7 @@ matrix_backup_borg_location_source_directories: []
 
 # postgres db backup
 matrix_backup_borg_postgresql_enabled: true
-matrix_backup_borg_supported_postgres_versions: ['12', '13', '14']
+matrix_backup_borg_supported_postgres_versions: ['12', '13', '14', '15']
 matrix_backup_borg_postgresql_databases: []
 matrix_backup_borg_postgresql_databases_hostname: ''
 matrix_backup_borg_postgresql_databases_username: "matrix"