Require Vala 0.21.2
Require Vala 0.21.2

Related to #7486, forgot to bump up the valac version minimum.

abrauchli/shotwell,abrauchli/shotwell,abrauchli/shotwell,jimbonator/shotwell,ritchiewilson/shotwell,jimbonator/shotwell,ritchiewilson/shotwell,jimbonator/shotwell,ritchiewilson/shotwell