Upcoming release: Remove support for `pk:` prefix in post mentions

[ok300]

Address this TODO

pubky-nexus/nexus-watcher/src/events/handlers/post.rs

Lines 270 to 273 in 3a9e0f3

	// TODO Deprecate, drop support for pk: support in an upcoming release
	// Backwards compatibility: identify user references with "pk:" prefix
	put_mentioned_relationships_for_prefix(author_id, post_id, content, relationships, "pk:")
	.await?;

introduced in #703.