[MM-53466] Remove concurrency from data retention delete batching (#23927)

* Add TimeBetweenBatches config value for data retention

* adding feature flag for data retention concurrency
---------

Co-authored-by: Julien Tant <julien@craftyx.fr>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/feature_flags.go

@@ -62,6 +62,8 @@ type FeatureFlags struct {
 	DeprecateCloudFree bool
 
 	CloudReverseTrial bool
+
+	DataRetentionConcurrencyEnabled bool
 }
 
 func (f *FeatureFlags) SetDefaults() {
@@ -85,6 +87,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.WysiwygEditor = false
 	f.OnboardingTourTips = true
 	f.CloudReverseTrial = false
+	f.DataRetentionConcurrencyEnabled = true
 }
 
 func (f *FeatureFlags) Plugins() map[string]string {