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/oauth2.rs

@@ -41,6 +41,7 @@ struct Oauth2 {
 
 /// OAuth 2 Access Token Response
 #[derive(Debug, Deserialize)]
+#[allow(dead_code)]
 struct Response {
     // Should always be there according to: <https://www.oauth.com/oauth2-servers/access-tokens/access-token-response/>
     // but previous code handled its abscense.