dep: bump min supported version of libxml2 to 2.9.2

[flavorjones]

In #3232 I bumped it to 2.7.7 but it turns out ...

• 2.7.7 nanohttp.c doesn't compile on my dev machine
• 2.8.0 doesn't have PARSE_BIG_LINES, GNOME/libxml2@968a03a2 introduced it in 2.9.0
• 2.9.0 and 2.9.1 have the xpath optimization bug that ruined our CSS queries in :first-child broken with libxml 2.9.0 #829, GNOME/libxml2@b4bcba23 fixed it in 2.9.2

We should also bump the min recommended version to something like 2.12 because Nick's been doing so much great work the last couple of years.