curldown: make 'added-in:' a mandatory header field

- generate AVAILABILITY manpage sections automatically - for consistent
wording

- allows us to double-check against other d

curldown: make 'added-in:' a mandatory header field

- generate AVAILABILITY manpage sections automatically - for consistent
wording

- allows us to double-check against other documumentation (symbols-in-versions
etc)

- enables proper automation/scripting based on this data

- lots of them were wrong or missing in the manpages

- several of them repeated (sometimes mismatching) backend support info

Add test 1488 to verify "added-in" version numbers against
symbols-in-versions.

Closes #14217

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