configure: add upper version constraint on libplacebo

Open Niklas Haas requested to merge haasn/vlc:vlc3_pl6 into 3.0.x

Libplacebo v6 substantially reworks the public API related to color spaces, tone mapping, and more - and also requires GLSL 130 as a minimum version, as well as access to OpenGL-backed textures to do any nontrivial form of color mapping.

Not only does this result in compilation failures, but it also totally prevents basic operation. These problems can be addressed on VLC v4, but backporting all of that to VLC v3 would be prohitively costly. As such, simply prevent VLC v3 from being combined with libplacebo v6, intentionallly or otherwise.

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