Fix some issues reported by lintian

Fix some issues reported by lintian

• Trim trailing whitespace. (trailing-whitespace)

• Remove 1 obsolete maintscript entry.

• Bump debhelper from old 12 to 13. (package-uses-old-debhelper-compat-version)

• Update standards version to 4.6.1, no changes needed. (out-of-date-standards-version)

This merge proposal was created automatically by the Janitor bot. For more information, including instructions on how to disable these merge proposals, see https://janitor.debian.net/lintian-fixes.

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/etckeeper/e7fe807e-d653-4e5b-8901-4aba20309f55.

These changes have no impact on the binary debdiff.

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