Fix Rust 1.57 compiler and clippy warnings (#2863)

* Remove unused os_name

* Disable clippy::manual_split_once lint

It's suggestions in stable 1.57 Rust are incorrect: https://github.com/rust-lang/rust-clippy/issues/7889

* Allow unused fields in OAuth2 response

* Fix clippy warning about `select_pk_for_encryption`

src/test_utils.rs

@@ -99,7 +99,7 @@ impl TestContext {
             let mut context_names = CONTEXT_NAMES.write().unwrap();
             context_names.insert(id, name);
         }
-        let ctx = Context::new("FakeOS".into(), dbfile.into(), id)
+        let ctx = Context::new(dbfile.into(), id)
             .await
             .expect("failed to create context");