community/stb: move from testing
This is needed for !40744 (merged) as curaengine
(which is in community) now needs system installed stb
in order to build whereas before it used an in-tree vendored `stb.
This is needed for !40744 (merged) as curaengine
(which is in community) now needs system installed stb
in order to build whereas before it used an in-tree vendored `stb.