Update go.mod and go.work to 1.21

Code in build/soong is using new builtins like max, update go.work
and go.mod to require 1.21 so that go test ./... works.

Fixes:
ui/status/ninja.go:193:25: max requires go1.21 or later (-lang was set to go1.20; check go.mod)

Test: go test ./...
Change-Id: I38f1a2298937db0d7c741b2183540fa3d02dac79