overview: remove bottom margin under motd
overview: remove bottom margin under motd

Instead of relying on margin, content in the overview grid relies on the
grid gap for spacing.

Fixes #13884
Closes #13919
martinpitt/cockpit,cockpituous/cockpit,garrett/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,deryni/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpit-project/cockpit,cockpituous/cockpit,cockpituous/cockpit,deryni/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit,cockpituous/cockpit,deryni/cockpit,mvollmer/cockpit,garrett/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit