refactor: use slices package for sort (#10132)

The sort package is still used in places that were not trivial to
change. Since Go 1.21 slices package can be uswed for sort. See
https://go.dev/doc/go1.21#slices

### Purpose

Make some progress with the migration to a more up-to-date syntax.

cmd/syncthing/cli/index_dumpsize.go

@@ -7,9 +7,10 @@
 package cli
 
 import (
+	"cmp"
 	"encoding/binary"
 	"fmt"
-	"sort"
+	"slices"
 
 	"github.com/syncthing/syncthing/lib/db"
 )
@@ -77,8 +78,8 @@ func indexDumpSize() error {
 		elems = append(elems, ele)
 	}
 
-	sort.Slice(elems, func(i, j int) bool {
-		return elems[i].size > elems[j].size
+	slices.SortFunc(elems, func(a, b sizedElement) int {
+		return cmp.Compare(b.size, a.size)
 	})
 	for _, ele := range elems {
 		fmt.Println(ele.key, ele.size)

cmd/syncthing/cli/index_idxck.go

@@ -8,10 +8,11 @@ package cli
 
 import (
 	"bytes"
+	"cmp"
 	"encoding/binary"
 	"errors"
 	"fmt"
-	"sort"
+	"slices"
 
 	"google.golang.org/protobuf/proto"
 
@@ -207,11 +208,11 @@ func indexCheck() (err error) {
 
 	// Aggregate the ranges of missing sequence entries, print them
 
-	sort.Slice(missingSeq, func(a, b int) bool {
-		if missingSeq[a].folder != missingSeq[b].folder {
-			return missingSeq[a].folder < missingSeq[b].folder
+	slices.SortFunc(missingSeq, func(a, b sequenceKey) int {
+		if a.folder != b.folder {
+			return cmp.Compare(a.folder, b.folder)
 		}
-		return missingSeq[a].sequence < missingSeq[b].sequence
+		return cmp.Compare(a.sequence, b.sequence)
 	})
 
 	var folder uint32

cmd/syncthing/main.go

@@ -23,7 +23,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"runtime/pprof"
-	"sort"
+	"slices"
 	"strconv"
 	"strings"
 	"syscall"
@@ -443,7 +443,7 @@ func debugFacilities() string {
 			maxLen = len(name)
 		}
 	}
-	sort.Strings(names)
+	slices.Sort(names)
 
 	// Format the choices
 	b := new(bytes.Buffer)