Refactor: func StringInSlice to slices.Contains (#24262)

* Refactor: Use generic to change func StringInSlice to Contains

* Refactor: Use generic to change func stringNotInSlice to Contains

Make utils.go (dir server/public/utils) and func Contains

* Refactor: Move func Contains from channels/utils to public/utils

Move func Contains from channels/utils to public/utils

Fix import declarations line

* Docs: Add a description of the Contains function

* Test: add TestContains

Add a test code for a Contain function

server/channels/app/plugin_signature.go

@@ -15,6 +15,8 @@ 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"
 )
 
@@ -46,7 +48,7 @@ func (a *App) AddPublicKey(name string, key io.Reader) *model.AppError {
 	}
 
 	a.UpdateConfig(func(cfg *model.Config) {
-		if !utils.StringInSlice(name, cfg.PluginSettings.SignaturePublicKeyFiles) {
+		if !pUtils.Contains(cfg.PluginSettings.SignaturePublicKeyFiles, name) {
 			cfg.PluginSettings.SignaturePublicKeyFiles = append(cfg.PluginSettings.SignaturePublicKeyFiles, name)
 		}
 	})