Dear Maintainer,

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   * What led up to the situation?
   * What exactly did you do (or not do) that was effective (or
     ineffective)?
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   * What outcome did you expect instead?

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Running backupninja creates warnings-logs while removing old backups :
WARNING: this command line interface is deprecated and will disappear, start using the new one as described with '--new --help'

In the FAQ this is mentioned for another option:
https://rdiff-backup.net/FAQ.html#why_does_rdiff_backup_complain_about_command_line_interface_being_deprecated_even_though_im_using_the_new_syntax

Chaning the line 177 in /usr/share/backupninja/rdiff:

	removestr="$RDIFFBACKUP $options --force --remove-older-than $keep "
to
	removestr="$RDIFFBACKUP $options remove increments --older-than $keep "

seams to help.

Some additional information about this bug :
* bug appeared with Debian bookworm / rdiff-backup 2.2.x
* rdiff-backup documentation about the CLI syntax change is available at
https://github.com/rdiff-backup/rdiff-backup/blob/master/docs/migration.adoc#migration-from-old-to-new-cli
* backupninja git tracker is at https://0xacab.org/liberate/backupninja but
apparently doesn't allow registering for an account. I tried with several
email addresses and always got « Email is not allowed for sign-up »
* if you specify additional options for rdiff-backup in your backupninja
config, for example « options = --no-acls », these options may not be valid
for the "remove increments" command. in my case, I had to strip this option
:
   removestr="$RDIFFBACKUP ${options/--no-acls/} remove increments
--older-than $keep "