refactor: use slices package for sorting (#10136)

Few more complicated usages of the sort packages are left.

### Purpose

Make progress towards replacing the sort package with slices package.

lib/config/config.go

@@ -18,7 +18,6 @@ import (
 	"os"
 	"reflect"
 	"slices"
-	"sort"
 	"strconv"
 	"strings"
 
@@ -338,8 +337,8 @@ func (cfg *Configuration) prepareDeviceList() map[protocol.DeviceID]*DeviceConfi
 	// - sorted by ID
 	// Happen before preparting folders as that needs a correct device list.
 	cfg.Devices = ensureNoDuplicateOrEmptyIDDevices(cfg.Devices)
-	sort.Slice(cfg.Devices, func(a, b int) bool {
-		return cfg.Devices[a].DeviceID.Compare(cfg.Devices[b].DeviceID) == -1
+	slices.SortFunc(cfg.Devices, func(a, b DeviceConfiguration) int {
+		return a.DeviceID.Compare(b.DeviceID)
 	})
 
 	// Build a list of available devices

lib/config/deviceconfiguration.go

@@ -8,7 +8,7 @@ package config
 
 import (
 	"fmt"
-	"sort"
+	"slices"
 
 	"github.com/syncthing/syncthing/lib/protocol"
 )
@@ -100,8 +100,8 @@ func sortedObservedFolderSlice(input map[string]ObservedFolder) []ObservedFolder
 	for _, folder := range input {
 		output = append(output, folder)
 	}
-	sort.Slice(output, func(i, j int) bool {
-		return output[i].Time.Before(output[j].Time)
+	slices.SortFunc(output, func(a, b ObservedFolder) int {
+		return a.Time.Compare(b.Time)
 	})
 	return output
 }

lib/config/folderconfiguration.go

@@ -15,7 +15,7 @@ import (
 	"fmt"
 	"path"
 	"path/filepath"
-	"sort"
+	"slices"
 	"strings"
 	"time"
 
@@ -291,8 +291,8 @@ func (f *FolderConfiguration) prepare(myID protocol.DeviceID, existingDevices ma
 	f.Devices = ensureDevicePresent(f.Devices, myID)
 	f.Devices = ensureNoUntrustedTrustingSharing(f, f.Devices, existingDevices)
 
-	sort.Slice(f.Devices, func(a, b int) bool {
-		return f.Devices[a].DeviceID.Compare(f.Devices[b].DeviceID) == -1
+	slices.SortFunc(f.Devices, func(a, b FolderDeviceConfiguration) int {
+		return a.DeviceID.Compare(b.DeviceID)
 	})
 
 	if f.RescanIntervalS > MaxRescanIntervalS {