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.

ui/widgets/albumfilterbutton.go

@@ -2,7 +2,7 @@ package widgets
 
 import (
 	"fmt"
-	"sort"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -49,7 +49,7 @@ func NewAlbumFilterButton(filter *mediaprovider.AlbumFilter, fetchGenresFunc fun
 			genreNames := sharedutil.MapSlice(genres, func(g *mediaprovider.Genre) string {
 				return g.Name
 			})
-			sort.Strings(genreNames)
+			slices.Sort(genreNames)
 			a.genreListChan <- genreNames
 		}
 	}()

ui/widgets/gridviewitem.go

@@ -3,9 +3,9 @@ package widgets
 import (
 	"image"
 	"image/color"
+	"slices"
 
 	"github.com/dweymouth/supersonic/res"
-	"github.com/dweymouth/supersonic/sharedutil"
 	"github.com/dweymouth/supersonic/ui/layouts"
 	"github.com/dweymouth/supersonic/ui/util"
 
@@ -205,7 +205,7 @@ func (g *GridViewItem) createContainer() {
 }
 
 func (g *GridViewItem) NeedsUpdate(model GridViewItemModel) bool {
-	return g.itemID != model.ID || !sharedutil.SliceEqual(g.secondaryIDs, model.SecondaryIDs)
+	return g.itemID != model.ID || !slices.Equal(g.secondaryIDs, model.SecondaryIDs)
 }
 
 func (g *GridViewItem) Update(model GridViewItemModel) {

ui/widgets/tracklist.go

@@ -3,6 +3,7 @@ package widgets
 import (
 	"fmt"
 	"log"
+	"slices"
 	"sort"
 	"strconv"
 	"strings"
@@ -271,7 +272,7 @@ func (t *Tracklist) Sorting() TracklistSort {
 func (t *Tracklist) SetSorting(sorting TracklistSort) {
 	if sorting.ColumnName == "" {
 		// nil case - reset current sort
-		if sharedutil.SliceContains(columns, t.sorting.ColumnName) {
+		if slices.Contains(columns, t.sorting.ColumnName) {
 			t.hdr.SetSorting(ListHeaderSort{ColNumber: ColNumber(t.sorting.ColumnName), Type: SortNone})
 		}
 		return
@@ -667,7 +668,7 @@ func (t *Tracklist) lenTracks() int {
 }
 
 func ColNumber(colName string) int {
-	i := sharedutil.IndexOf(columns, colName)
+	i := slices.Index(columns, colName)
 	if i < 0 {
 		log.Printf("error: Tracklist: invalid column name %s", colName)
 	}