Fix "removing leading /" stderr output by tar in backup-and-maintenance.sh

[Yannik]

Description

This fix removes the "tar: Removing leading `/' from member names" stderr output which otherwise occurs on every run

Previous stderr output:

$ /usr/local/pf/addons/backup-and-maintenance.sh >/dev/null
tar: Removing leading `/' from member names
$

New stderr output:

$ /usr/local/pf/addons/backup-and-maintenance.sh >/dev/null
$

Impacts

This does not change functionality.

Issue

This is a major step for #1415, as stderr output is seen as "faulty"

Delete branch after merge

(REQUIRED)
YES

Checklist

(REQUIRED) - [yes, no or n/a]

• [n/a] Document the feature
• [n/a] Add unit tests
• [n/a] Add acceptance tests (TestLink)

NEWS file entries

(REQUIRED, but may be optional...)

Enhancements

• Fix "removing leading /" stderr output by tar in backup-and-maintenance.sh

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