@Nullable
    @UnstableApi
    default Boolean defaultUseHttp2WithoutAlpn() {
        return null;
    }
Returns the default value of the {@link ClientFactoryBuilder#useHttp2WithoutAlpn(boolean)} option.
If enabled, even when ALPN negotiation fails client will try to attempt upgrade to HTTP/2 when needed.
This will be either HTTP/2 connection preface or HTTP/1-to-2 upgrade request,
depending on {@link ClientFactoryBuilder#useHttp2Preface(boolean)} setting.
If disabled, when ALPN negotiation fails client will also fail in case HTTP/2 was required.
{@link ClientFactoryBuilder#useHttp2WithoutAlpn(boolean)}.

<p>This flag is disabled by default. Specify the
{@code -Dcom.linecorp.armeria.defaultUseHttp2WithoutAlpn=true} JVM option to enable it.
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java