Consider using restic instead of duplicity

[php-coder]

See https://restic.net and https://rclone.org + https://habr.com/ru/companies/ruvds/articles/797457/ + https://jvns.ca/til/restic-for-backing-up-sqlite-dbs/

Motivation:

• It seems like duplicity (with webdavs protocol) doesn't support strong encryption and it stops working when a server uses it (this fact lead to a broken backup twice)
• the password may be revealed with ps utility (see #739)
• for some reason duplicity doesn't remove stale backups and it's not clear from the first sight why

Advantages of Restic isn't known yet but if it doesn't have aforementioned issues it worth replacement anyway.

Idea from @asm0dey