fix: JWTTokenProvider has an incorrect value for the returned authorization header

more info:
https://docs.github.com/en/free-pro-team@latest/developers/apps/authenticating-with-github-apps#authenticating-as-a-github-app

src/main/java/org/kohsuke/github/extras/auth/JWTTokenProvider.java

@@ -62,7 +62,7 @@ public class JWTTokenProvider implements AuthorizationProvider {
             if (Instant.now().isAfter(validUntil)) {
                 token = refreshJWT();
             }
-            return token;
+            return String.format("Bearer %s", token);
         }
     }