Can't install dnsmasq-full, missing libubus20210603

Known error. Due to update in ubus & libubus version after rc2.
See 21.02.0-rc2 - Build Error on libubus20210215

Use the 21.02 snapshot firmware builds, or the rc3 builds (which are currently being compiled).

To decrease the probability of similar errors in future, libubus was moved to be built together with the firmware images (just before the rc3). And that move actually causes the specific error that you are seeing. rc3 should fix it.