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

lib/multipart/multipart.go

@@ -62,10 +62,7 @@ func UploadMultipart(ctx context.Context, src fs.ObjectInfo, in io.Reader, opt U
 	}
 
 	// make concurrency machinery
-	concurrency := info.Concurrency
-	if concurrency < 1 {
-		concurrency = 1
-	}
+	concurrency := max(info.Concurrency, 1)
 	tokens := pacer.NewTokenDispenser(concurrency)
 
 	uploadCtx, cancel := context.WithCancel(ctx)