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

fs/asyncreader/asyncreader_test.go

@@ -151,18 +151,18 @@ func TestAsyncReaderSizes(t *testing.T) {
 	var texts [31]string
 	str := ""
 	all := ""
-	for i := 0; i < len(texts)-1; i++ {
+	for i := range len(texts) - 1 {
 		texts[i] = str + "\n"
 		all += texts[i]
 		str += string(rune(i)%26 + 'a')
 	}
 	texts[len(texts)-1] = all
 
-	for h := 0; h < len(texts); h++ {
+	for h := range len(texts) {
 		text := texts[h]
-		for i := 0; i < len(readMakers); i++ {
-			for j := 0; j < len(bufreaders); j++ {
-				for k := 0; k < len(bufsizes); k++ {
+		for i := range readMakers {
+			for j := range bufreaders {
+				for k := range bufsizes {
 					for l := 1; l < 10; l++ {
 						readmaker := readMakers[i]
 						bufreader := bufreaders[j]
@@ -192,18 +192,18 @@ func TestAsyncReaderWriteTo(t *testing.T) {
 	var texts [31]string
 	str := ""
 	all := ""
-	for i := 0; i < len(texts)-1; i++ {
+	for i := range len(texts) - 1 {
 		texts[i] = str + "\n"
 		all += texts[i]
 		str += string(rune(i)%26 + 'a')
 	}
 	texts[len(texts)-1] = all
 
-	for h := 0; h < len(texts); h++ {
+	for h := range len(texts) {
 		text := texts[h]
-		for i := 0; i < len(readMakers); i++ {
-			for j := 0; j < len(bufreaders); j++ {
-				for k := 0; k < len(bufsizes); k++ {
+		for i := range readMakers {
+			for j := range bufreaders {
+				for k := range bufsizes {
 					for l := 1; l < 10; l++ {
 						readmaker := readMakers[i]
 						bufreader := bufreaders[j]