[MM-57356] Make use of go1.21 features (#26620)

server/channels/app/plugin_signature.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"net/http"
 	"path/filepath"
+	"slices"
 
 	"github.com/pkg/errors"
 	"golang.org/x/crypto/openpgp"       //nolint:staticcheck
@@ -15,7 +16,6 @@ import (
 
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
-	pUtils "github.com/mattermost/mattermost/server/public/utils"
 
 	"github.com/mattermost/mattermost/server/v8/channels/utils"
 )
@@ -48,7 +48,7 @@ func (a *App) AddPublicKey(name string, key io.Reader) *model.AppError {
 	}
 
 	a.UpdateConfig(func(cfg *model.Config) {
-		if !pUtils.Contains(cfg.PluginSettings.SignaturePublicKeyFiles, name) {
+		if !slices.Contains(cfg.PluginSettings.SignaturePublicKeyFiles, name) {
 			cfg.PluginSettings.SignaturePublicKeyFiles = append(cfg.PluginSettings.SignaturePublicKeyFiles, name)
 		}
 	})