Update mockito requirement from 0.30 to 0.31 (#666)

Author: dependabot[bot]

sdk/security_keyvault/Cargo.toml

@@ -27,6 +27,6 @@ azure_core = { path = "../core", version = "0.1" }
 [dev-dependencies]
 oauth2 = "4.0.0"
 azure_identity = { path = "../identity", version = "0.1" }
-mockito = "0.30"
+mockito = "0.31"
 async-trait = "0.1"
 tokio = { version = "1.0", features = ["full"] }