devel/glib20: revert update to 2.68.2

This reverts commit 507359c509389602b4060b2c5e203c99911c3578.

devel/glib20 was touched without desktop@ approval and properly passed exp-run which caused
a massive build failures for critical parts of portstree such as *webkit*, grstreamer* and
many others due to incompatible changes in the new glib API.

Revert, bump PORTEPOCH and freeze at 2.66.* branch till all consumers will be adopted to new API.

With hat: ports-secteam, desktop
Pointy hat to: nc@