Bump Golang to 1.21

[gliptak]

should golintci/static-analysis exclude dependent packages?

https://github.com/wagoodman/dive/actions/runs/7788571006/job/21238257819?pr=505

./.tmp/golangci-lint run --tests=false --timeout=2m --config .golangci.yaml
Error: ../../../../../opt/hostedtoolcache/go/1.21.6/x64/src/net/http/internal/chunked.go:7[9](https://github.com/wagoodman/dive/actions/runs/7788571006/job/21238257819?pr=505#step:4:10):14: undefined: max (typecheck)
    cr.excess = max(cr.excess, 0)
                ^
Error: ../../../go/pkg/mod/github.com/docker/docker@v24.0.7+incompatible/client/request.go:9:2: "net" imported and not used (typecheck)
    "net"
    ^

[gliptak]

@wagoodman please review