⬆️ Requre zvariant 3.15

In 2.6.0, we ended up using API from zvariant 3.15 and didn't bump our
requirement. Let's do that.

Fixes #667.

zbus_names/Cargo.toml

@@ -14,7 +14,7 @@ readme = "README.md"
 
 [dependencies]
 serde = { version = "1.0", features = ["derive"] }
-zvariant = { path = "../zvariant", version = "3.10", default-features = false, features = ["enumflags2"] }
+zvariant = { path = "../zvariant", version = "3.15", default-features = false, features = ["enumflags2"] }
 static_assertions = "1.1.0"
 
 [package.metadata.docs.rs]