`ConnectionSpec` class.

    To disable TLS fallback:

    ```java
    client.setConnectionSpecs(Arrays.asList(
        ConnectionSpec.MODERN_TLS, ConnectionSpec.CLEARTEXT));
    ```

    To disable cleartext connections, permitting `https` URLs only:

    ```java
    client.setConnectionSpecs(Arrays.asList(
        ConnectionSpec.MODERN_TLS, ConnectionSpec.COMPATIBLE_TLS));
    ```