[charms] Nominate Martin Kalcok for charms-core

Hello all,

I would like to nominate Martin Kalcok for charms-core. He has been part of the extended charms teams for some time now, and has provided many contributions to OpenStack, Ceph and OVN charms [0]. He is diligent and thorough in his contributions, and has recently become a full time member of the focused OVN team.

0: https://review.opendev.org/q/owner:martin.kalcok%2540canonical.com