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

backend/pcloud/pcloud.go

@@ -990,10 +990,7 @@ func (f *Fs) About(ctx context.Context) (usage *fs.Usage, err error) {
 	if err != nil {
 		return nil, err
 	}
-	free := q.Quota - q.UsedQuota
-	if free < 0 {
-		free = 0
-	}
+	free := max(q.Quota-q.UsedQuota, 0)
 	usage = &fs.Usage{
 		Total: fs.NewUsageValue(q.Quota),     // quota of bytes that can be used
 		Used:  fs.NewUsageValue(q.UsedQuota), // bytes in use
@@ -1324,7 +1321,7 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	if err != nil {
 		// sometimes pcloud leaves a half complete file on
 		// error, so delete it if it exists, trying a few times
-		for i := 0; i < 5; i++ {
+		for range 5 {
 			delObj, delErr := o.fs.NewObject(ctx, o.remote)
 			if delErr == nil && delObj != nil {
 				_ = delObj.Remove(ctx)

backend/pcloud/writer_at.go

@@ -37,7 +37,7 @@ func (c *writerAt) Close() error {
 	}
 	sizeOk := false
 	sizeLastSeen := int64(0)
-	for retry := 0; retry < 5; retry++ {
+	for retry := range 5 {
 		fs.Debugf(c.remote, "checking file size: try %d/5", retry)
 		obj, err := c.fs.NewObject(c.ctx, c.remote)
 		if err != nil {