With vSphere 7.0.u2 VMware introduced a internal KMS which comes with vCenter. With a valid subscription you can upgrade your VCSA from 6.7 to 7.x. You dont need to upgrade the Hosts. (Keep in mind that support end for 6.x is 10/2022).
HINT: There are some post here in the forum which say that they cant activate the KMS within vCenter Essentials. So you need to investigate here.
Regards,
Joerg