Add deprecation entry for augur export v1

DEPRECATED.md

@@ -3,3 +3,11 @@
 These features are deprecated, which means they are no longer maintained and
 will go away in a future major version of Augur. They are currently still
 available for backwards compatibility, but should not be used in new code.
+
+## `augur export v1`
+
+*Deprecated in version 22.2.0 (July 2023). Planned for [removal](https://github.com/nextstrain/augur/issues/1266)
+January 2024 or after.*
+
+`augur export v2` was introduced in Augur version 6.0.0. Migrate by following
+the [official guide](https://docs.nextstrain.org/projects/augur/en/stable/releases/migrating-v5-v6.html).