@UnstableApi
    @Nullable
    default Boolean defaultPreferHttp1() {
        return null;
    }
Returns the default value of the {@link ClientFactoryBuilder#preferHttp1(boolean)} option.
If enabled, the client will not attempt to upgrade to HTTP/2 for {@link SessionProtocol#HTTP} and
{@link SessionProtocol#HTTPS}.

<p>Note that this option has no effect if a user specified the value explicitly via
{@link ClientFactoryBuilder#preferHttp1(boolean)}.

<p>This flag is disabled by default. Specify the
{@code -Dcom.linecorp.armeria.defaultPreferHttp1=true} JVM option to enable it.
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java