Migrate to borg 2

"It looks like the migration to borg 2.x will require some care, coordination, and extra work." -- @intrigeri

See:

• transition plan for borgbackup 2.x
• ITP: borgbackup2 -- version 2.x of a deduplicating and compressing backup program

Notes:

• As per the current proposed plan, backups should not break after a dist-upgrade, but transitioning to the newer version of borg will need manual intervention.
• borgbackup (a.k.a. "borg1") is currently in trixie/testing, so we have some time to tackle this and don't need to rush.

Note: For S11, this fits in:

• B.2 - Keep our infrastructure up-to-date and secure: Using up-to-date cryptographic tools to protect our backups is fundamental to avoid risks such as such as losing data or leaking credentials.