Fix mistaken std::move assignment.

[bangerth]

This fixes #16332. The issue here is that the assignment

temperature_solution     = tmp[0];

doesn't just copy the rhs to the lhs, but because the lhs is a ghosted vector and the rhs is not, it also imports elements. We do not get these semantics if we make this into a move assignment

temperature_solution     = std::move(tmp[0]);

as I had mistakenly done in #15438.