Major Version Upgrade on Debian

• make a snapshot/backup of the machine
• check the running clusters using pg_lsclusters command
  
• drop the new cluster (the one running the target version) sudo -u postgres pg_dropcluster --stop 15 main
• (optional) if you are ssh-ing from a macOS/Item2 the LC_CTYPE is wrongly set
  • run export LC_CTYPE=en_US.UTF-8 export LC_ALL=en_US.UTF-8
• upgrade the old cluster sudo -u postgres pg_upgradecluster 13 main
• check the running clusters using pg_lsclusters command
• drop the old cluster (be sure the upgrade was successful) sudo -u postgres pg_dropcluster 13 main
• uninstall old postgresql packages