MM-36271: Bump major version to 6.0 (#17973)

https://mattermost.atlassian.net/browse/MM-36271

```release-note
We bump the major version to 6.0
```

store/layer_generators/opentracing_layer.go.tmpl

@@ -9,9 +9,9 @@ package opentracinglayer
 import (
 	"context"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/services/tracing"
-	"github.com/mattermost/mattermost-server/v5/store"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/services/tracing"
+	"github.com/mattermost/mattermost-server/v6/store"
 	"github.com/opentracing/opentracing-go/ext"
 	spanlog "github.com/opentracing/opentracing-go/log"
 )

store/layer_generators/retry_layer.go.tmpl

@@ -10,8 +10,8 @@ import (
 	"context"
 
 	"github.com/lib/pq"
-	"github.com/mattermost/mattermost-server/v5/model"
-    "github.com/mattermost/mattermost-server/v5/store"
+	"github.com/mattermost/mattermost-server/v6/model"
+    "github.com/mattermost/mattermost-server/v6/store"
     "github.com/pkg/errors"
 	"github.com/go-sql-driver/mysql"
 )

store/layer_generators/timer_layer.go.tmpl

@@ -10,9 +10,9 @@ import (
 	"context"
 	timemodule "time"
 
-    "github.com/mattermost/mattermost-server/v5/einterfaces"
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/store"
+    "github.com/mattermost/mattermost-server/v6/einterfaces"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/store"
 )
 
 type {{.Name}} struct {