misc: Upgrade Go to v1.21

Upgrade to 1.21 as 1.20 is now unmaintained.

Most changes are related to the new `slices` package from standard
library, which can replace some existing custom functions.

backend/app.go

@@ -8,12 +8,12 @@ import (
 	"os"
 	"path"
 	"reflect"
+	"slices"
 	"time"
 
 	"github.com/dweymouth/supersonic/backend/player"
 	"github.com/dweymouth/supersonic/backend/player/mpv"
 	"github.com/dweymouth/supersonic/backend/util"
-	"github.com/dweymouth/supersonic/sharedutil"
 	"github.com/fsnotify/fsnotify"
 	"github.com/google/uuid"
 
@@ -233,7 +233,7 @@ func (a *App) setupMPV() error {
 	a.LocalPlayer.SetAudioDevice(desiredDevice)
 
 	rgainOpts := []string{ReplayGainNone, ReplayGainAlbum, ReplayGainTrack, ReplayGainAuto}
-	if !sharedutil.SliceContains(rgainOpts, a.Config.ReplayGain.Mode) {
+	if !slices.Contains(rgainOpts, a.Config.ReplayGain.Mode) {
 		a.Config.ReplayGain.Mode = ReplayGainNone
 	}
 	mode := player.ReplayGainNone

backend/mediaprovider/helpers/iterators.go

@@ -64,9 +64,7 @@ func (r *baseIter[T]) Next() *T {
 		}
 		r.serverPos += len(items)
 		if !r.filter.IsNil() {
-			items = sharedutil.FilterSlice(items, func(al *T) bool {
-				return r.filter.Matches(al)
-			})
+			items = sharedutil.FilterSlice(items, r.filter.Matches)
 		}
 		r.prefetched = items
 		if len(items) > 0 {

backend/util/util.go

@@ -5,15 +5,6 @@ import (
 	"os"
 )
 
-// Return a slice of range [0, n)
-func Range(n int) []int {
-	s := make([]int, n)
-	for i := 0; i < n; i++ {
-		s[i] = i
-	}
-	return s
-}
-
 func CopyFile(srcPath, dstPath string) error {
 	fin, err := os.Open(srcPath)
 	if err != nil {