[youtube] Fix age gate detection

[random-nick]

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YouTube apparently renders errors using javascript now. I used the meta tag for age restriction as a workaround.

Closes #25954

[adrianheine]

This fixes the currently-failing TestAgeRestriction.test_youtube.

[jet082]

I can confirm that this seems to fix the age gate detection. However, it does not yet detect automatic subtitles for age-gated content.

[blackjack4494]

@random-nick needs a little rework. Take this video as an example 7takIh1nK0s. A test could be added for this.
You can find more information here.
blackjack4494#54

Your approach assumes you can get information for age gated videos without being logged in. But like I mentioned in the post at the end this case is special. It requires you to be logged in to watch it and in addition it is age gated. If this combination is common - I have no clue. Who knows who ultimately decided that this video is age gated. Could be the community, the uploader or youtube itself or a mix.

@dstftw