Updated version to 0.21.0
Updated version to 0.21.0

The version tries to follow that of libgit2 somewhat. Let's see if it
works out.

pav0n/libqgit2,pav0n/libqgit2,KDE/libqgit2,syntheticpp/libqgit2,syntheticpp/libqgit2,KDE/libqgit2