Fix some issues reported by lintian

Fix some issues reported by lintian

• Update standards version to 4.6.1, no changes needed. (out-of-date-standards-version)
• Remove field Priority on binary package libident-dev that duplicates source. (installable-field-mirrors-source)

This merge proposal was created by the Janitor bot, and it will automatically rebase or close this proposal as appropriate when the target branch changes. Any comments you leave here will be read by the Janitor's maintainers.

Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/libident/100edba4-aabd-49df-a5ab-5e952185637d.

These changes have no impact on the binary debdiff.

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