Fix some issues reported by lintian

Fix some issues reported by lintian

• Fix day-of-week for changelog entry 4.0.0.final-1. (debian-changelog-has-wrong-day-of-week)
• Update standards version to 4.4.1, no changes needed. (out-of-date-standards-version)
• Bump debhelper from old 11 to 12. (package-uses-old-debhelper-compat-version)

This merge proposal was created automatically by the Janitor bot.

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/libxfce4util/ae852490-ca03-4827-b0cb-3b70b4dd3e15.

These changes have no impact on the binary debdiff.

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