all: Remove crypto/md5 (#7493)

This is a mostly pointless change to make security scanners and static
analysis tools happy, as they all hate seeing md5. None of our md5 uses
were security relevant, but still. Only visible effect of this change is
that our temp file names for very long file names become slightly longer
than they were previously...

lib/fs/tempname.go

@@ -7,11 +7,12 @@
 package fs
 
 import (
-	"crypto/md5"
 	"fmt"
 	"path/filepath"
 	"runtime"
 	"strings"
+
+	"github.com/syncthing/syncthing/lib/sha256"
 )
 
 const (
@@ -50,9 +51,7 @@ func TempNameWithPrefix(name, prefix string) string {
 	tdir := filepath.Dir(name)
 	tbase := filepath.Base(name)
 	if len(tbase) > maxFilenameLength {
-		hash := md5.New()
-		hash.Write([]byte(name))
-		tbase = fmt.Sprintf("%x", hash.Sum(nil))
+		tbase = fmt.Sprintf("%x", sha256.Sum256([]byte(name)))
 	}
 	tname := fmt.Sprintf("%s%s.tmp", prefix, tbase)
 	return filepath.Join(tdir, tname)