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/box/upload.go

@@ -105,7 +105,7 @@ func (o *Object) commitUpload(ctx context.Context, SessionID string, parts []api
 	const defaultDelay = 10
 	var tries int
 outer:
-	for tries = 0; tries < maxTries; tries++ {
+	for tries = range maxTries {
 		err = o.fs.pacer.Call(func() (bool, error) {
 			resp, err = o.fs.srv.CallJSON(ctx, &opts, &request, nil)
 			if err != nil {
@@ -203,7 +203,7 @@ func (o *Object) uploadMultipart(ctx context.Context, in io.Reader, leaf, direct
 	errs := make(chan error, 1)
 	var wg sync.WaitGroup
 outer:
-	for part := 0; part < session.TotalParts; part++ {
+	for part := range session.TotalParts {
 		// Check any errors
 		select {
 		case err = <-errs:
@@ -211,10 +211,7 @@ outer:
 		default:
 		}
 
-		reqSize := remaining
-		if reqSize >= chunkSize {
-			reqSize = chunkSize
-		}
+		reqSize := min(remaining, chunkSize)
 
 		// Make a block of memory
 		buf := make([]byte, reqSize)