Upgrade client version to '0.4.0'

This makes clear that the previous is no more fully compatible with the aggregator
running the new 'zstandard' compression format.

Author: jpraynaud

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "mithril-client"
-version = "0.3.45"
+version = "0.4.0"
 description = "A Mithril Client"
 authors = { workspace = true }
 edition = { workspace = true }