www/webkit2-gtk: consolidate, flavourise, update to 2.38.6

[vishwin]

Summary

Same codebase, but GTK and libsoup combinations result in different SONAMEs:

• GTK 3 + libsoup = libwebkit2gtk-4.0
• GTK 3 + libsoup3 = libwebkit2gtk-4.1
• GTK 4 + libsoup3 = libwebkit2gtk-5.0

Flavourise these three combinations.

Reported by: eduardo
PR: 275914
Co-authored by: vishwin

Test Plan

Not the latest upstream release. Stopgap to introduce flavours and allow consumers to use correct dependency combinations, as later versions need more work.

Switch www/epiphany and net/remmina to the 41 flavour

Also includes an upstream commit addressing textproc/libxml2 2.12 breakage

[vishwin]

UPDATING entry also to come.

[vishwin]

fix www/epiphany's corresponding devel/libsoup3 LIB_DEPENDS

[vishwin]

Abandoning, as this builds but doesn't function (JavaScriptCore thread indefinite wait condition). 2.42 WIP in a new review