Fix #280: Mark ReCaptchaV3 as a hidden field

[matthiask]

This skips outputting a label and improves behavior around error messages.

I omitted the changelog entry because it would conflict with #293

[Andrew-Chen-Wang]

thanks! will await others for feedback and double check later

[andre-dasilva]

This would be great. Instead of overriding is_hidden (which is a property),
it might be better to set the input_type = 'hidden'

[Stormheg]

@matthiask for your information, I've pushed to your branch and made the suggested change to set input_type="hidden" + added a test to check the label is omitted.

[matthiask]

Thanks!