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/dropbox/dbhash/dbhash.go

@@ -55,10 +55,7 @@ func (d *digest) Write(p []byte) (n int, err error) {
 	n = len(p)
 	for len(p) > 0 {
 		d.writtenMore = true
-		toWrite := bytesPerBlock - d.n
-		if toWrite > len(p) {
-			toWrite = len(p)
-		}
+		toWrite := min(bytesPerBlock-d.n, len(p))
 		_, err = d.blockHash.Write(p[:toWrite])
 		if err != nil {
 			panic(hashReturnedError)

backend/dropbox/dbhash/dbhash_test.go

@@ -11,7 +11,7 @@ import (
 
 func testChunk(t *testing.T, chunk int) {
 	data := make([]byte, chunk)
-	for i := 0; i < chunk; i++ {
+	for i := range chunk {
 		data[i] = 'A'
 	}
 	for _, test := range []struct {