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/operations/multithread.go

@@ -73,10 +73,7 @@ func (mc *multiThreadCopyState) copyChunk(ctx context.Context, chunk int, writer
 	if start >= mc.size {
 		return nil
 	}
-	end := start + mc.partSize
-	if end > mc.size {
-		end = mc.size
-	}
+	end := min(start+mc.partSize, mc.size)
 	size := end - start
 
 	fs.Debugf(mc.src, "multi-thread copy: chunk %d/%d (%d-%d) size %v starting", chunk+1, mc.numChunks, start, end, fs.SizeSuffix(size))
@@ -218,7 +215,7 @@ func multiThreadCopy(ctx context.Context, f fs.Fs, remote string, src fs.Object,
 	mc.acc = tr.Account(gCtx, nil)
 
 	fs.Debugf(src, "Starting multi-thread copy with %d chunks of size %v with %v parallel streams", mc.numChunks, fs.SizeSuffix(mc.partSize), concurrency)
-	for chunk := 0; chunk < mc.numChunks; chunk++ {
+	for chunk := range mc.numChunks {
 		// Fail fast, in case an errgroup managed function returns an error
 		if gCtx.Err() != nil {
 			break