Edit your /etc/apt/sources.list and replace http://ports.ubuntu.com/ubuntu-ports by http://old-releases.ubuntu.com/ubuntu/. Then, re-run the commands.
Step by step:
Save a backup of /etc/apt/sources.list (althought you can also comment lines with #, but a backup is better for your own reference to leave the system as it was, just in case)
Replace the URLs in /etc/apt/sources.list by using http://old-releases.ubuntu.com/ubuntu/ instead of http://ports.ubuntu.com/ubuntu-ports
Run sudo apt-get update, which should run fine now.
Run sudo apt-get dist-upgrade, which should also run fine now.
And finally, install apt install update-manager-core and run do-release-upgrade (a reboot might be needed before running this command)
Hopefully that should be it. Feel free to disable all failing sources in your sources.list until you successfully upgrade the system.
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