[manila][stable] Additions and Removals from stable-core

Hello Stackers, I'd like to add a couple of maintainers to manila-stable-maint: https://review.opendev.org/#/admin/groups/1099,members - dviroel (Douglas Viroel) - silvacarlos (Carlos Eduardo) Both Douglas and Carlos understand the stable branch policy [1] and adhere to it. Thanks! Goutham [1] https://docs.openstack.org/project-team-guide/stable-branches.html