plug-ins: we still depends on librsvg 2.40.x.

In commits a6756b51 and 23da44e3, newer API of librsvg were used, over our
current minimum requirement. The reason why we don't bump this requirement is
that librsvg 2.40.x are the latest versions which are full-C. While we have
nothing against Rust, it is not available on all existing platforms, and we
really want to have GIMP buildable everywhere, while also have SVG support
always present (even though it's a vector format, it's an important one in
graphics work nowadays).
See issue #6821.

So in the end, let's keep the old minimum requirement, but test against librsvg
version to compiler either with old API or new one. The changes are limited
enough that it's not too much a bother.