Add entry to changelog

changelog/unreleased/issue-1906

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+Feature: Add support for B2 application keys
+
+Restic can now use so-called "application keys" which can be created in the B2
+dashboard and were only introduced recently. In contrast to the "master key",
+such keys can be restricted to a specific bucket and/or path.
+
+https://github.com/restic/restic/issues/1906
+https://github.com/restic/restic/pull/1914