ci: assignees: set multiple assignees of same area

Noticing many PRs that wait too long in the queue although once of the
maintainers approved with asignees set to other maintainers.

This changes the current behavior of picking the first maintainer in the
list and assigning to them only, instead we assign to all maintainers of
the main area being changed.

Who ends up driving the PR to a mergeable state is then to the
maintainers and they can unassign/assign based on availability.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>