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/crypt/pkcs7/pkcs7.go

@@ -25,7 +25,7 @@ func Pad(n int, buf []byte) []byte {
 	}
 	length := len(buf)
 	padding := n - (length % n)
-	for i := 0; i < padding; i++ {
+	for range padding {
 		buf = append(buf, byte(padding))
 	}
 	if (len(buf) % n) != 0 {
@@ -54,7 +54,7 @@ func Unpad(n int, buf []byte) ([]byte, error) {
 	if padding == 0 {
 		return nil, ErrorPaddingTooShort
 	}
-	for i := 0; i < padding; i++ {
+	for i := range padding {
 		if buf[length-1-i] != byte(padding) {
 			return nil, ErrorPaddingNotAllTheSame
 		}