build: modernize Go usage

This commit modernizes Go usage. This was done with:

go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...

Then files needed to be `go fmt`ed and a few comments needed to be
restored.

The modernizations include replacing

- if/else conditional assignment by a call to the built-in min or max functions added in go1.21
- sort.Slice(x, func(i, j int) bool) { return s[i] < s[j] } by a call to slices.Sort(s), added in go1.21
- interface{} by the 'any' type added in go1.18
- append([]T(nil), s...) by slices.Clone(s) or slices.Concat(s), added in go1.21
- loop around an m[k]=v map update by a call to one of the Collect, Copy, Clone, or Insert functions from the maps package, added in go1.21
- []byte(fmt.Sprintf...) by fmt.Appendf(nil, ...), added in go1.19
- append(s[:i], s[i+1]...) by slices.Delete(s, i, i+1), added in go1.21
- a 3-clause for i := 0; i < n; i++ {} loop by for i := range n {}, added in go1.22

cmd/serve/docker/docker_test.go

@@ -49,7 +49,7 @@ func initialise(ctx context.Context, t *testing.T) (string, fs.Fs) {
 	return testDir, testFs
 }
 
-func assertErrorContains(t *testing.T, err error, errString string, msgAndArgs ...interface{}) {
+func assertErrorContains(t *testing.T, err error, errString string, msgAndArgs ...any) {
 	assert.Error(t, err)
 	if err != nil {
 		assert.Contains(t, err.Error(), errString, msgAndArgs...)
@@ -244,7 +244,7 @@ func newAPIClient(t *testing.T, host, unixPath string) *APIClient {
 	}
 }
 
-func (a *APIClient) request(path string, in, out interface{}, wantErr bool) {
+func (a *APIClient) request(path string, in, out any, wantErr bool) {
 	t := a.t
 	var (
 		dataIn  []byte