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/features.go

@@ -191,7 +191,7 @@ type Features struct {
 	// The result should be capable of being JSON encoded
 	// If it is a string or a []string it will be shown to the user
 	// otherwise it will be JSON encoded and shown to the user like that
-	Command func(ctx context.Context, name string, arg []string, opt map[string]string) (interface{}, error)
+	Command func(ctx context.Context, name string, arg []string, opt map[string]string) (any, error)
 
 	// Shutdown the backend, closing any background tasks and any
 	// cached connections.
@@ -209,7 +209,7 @@ func (ft *Features) Disable(name string) *Features {
 	}
 	v := reflect.ValueOf(ft).Elem()
 	vType := v.Type()
-	for i := 0; i < v.NumField(); i++ {
+	for i := range v.NumField() {
 		vName := vType.Field(i).Name
 		field := v.Field(i)
 		if strings.EqualFold(name, vName) {
@@ -238,7 +238,7 @@ func (ft *Features) Disable(name string) *Features {
 func (ft *Features) List() (out []string) {
 	v := reflect.ValueOf(ft).Elem()
 	vType := v.Type()
-	for i := 0; i < v.NumField(); i++ {
+	for i := range v.NumField() {
 		out = append(out, vType.Field(i).Name)
 	}
 	return out
@@ -250,7 +250,7 @@ func (ft *Features) Enabled() (features map[string]bool) {
 	v := reflect.ValueOf(ft).Elem()
 	vType := v.Type()
 	features = make(map[string]bool, v.NumField())
-	for i := 0; i < v.NumField(); i++ {
+	for i := range v.NumField() {
 		vName := vType.Field(i).Name
 		field := v.Field(i)
 		if field.Kind() == reflect.Func {
@@ -761,7 +761,7 @@ type Commander interface {
 	// The result should be capable of being JSON encoded
 	// If it is a string or a []string it will be shown to the user
 	// otherwise it will be JSON encoded and shown to the user like that
-	Command(ctx context.Context, name string, arg []string, opt map[string]string) (interface{}, error)
+	Command(ctx context.Context, name string, arg []string, opt map[string]string) (any, error)
 }
 
 // Shutdowner is an interface to wrap the Shutdown function