[styles] Format title of \codeblocktu using 'caption' package

[jensmaurer]

This restores the C++20 status of the formatting.

[jensmaurer]

Apparently, our prior attempt at redefining some internal stuff of the listings package got neutered by later versions of said package.

Use the right tool for the job instead.

[tkoeppe]

@jensmaurer Currently, this allows page breaks between the title and the code block. I couldn't find a general way to fix this easily and am adding a manual break hint before "Translation unit #2" in [basic.link]. I was trying adding a negative penalty before the title to encourage page breaks, but that ends up adding unnecessary breaks.

[jensmaurer]

Yeah, getting the page break penalties right is probable for another day.