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
```

testlib/cluster.go

@@ -6,8 +6,8 @@ package testlib
 import (
 	"sync"
 
-	"github.com/mattermost/mattermost-server/v5/einterfaces"
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/einterfaces"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type FakeClusterInterface struct {

testlib/helper.go

@@ -14,14 +14,14 @@ import (
 
 	"github.com/pkg/errors"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/services/searchengine"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
-	"github.com/mattermost/mattermost-server/v5/store"
-	"github.com/mattermost/mattermost-server/v5/store/searchlayer"
-	"github.com/mattermost/mattermost-server/v5/store/sqlstore"
-	"github.com/mattermost/mattermost-server/v5/store/storetest"
-	"github.com/mattermost/mattermost-server/v5/utils"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/services/searchengine"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/store"
+	"github.com/mattermost/mattermost-server/v6/store/searchlayer"
+	"github.com/mattermost/mattermost-server/v6/store/sqlstore"
+	"github.com/mattermost/mattermost-server/v6/store/storetest"
+	"github.com/mattermost/mattermost-server/v6/utils"
 )
 
 type MainHelper struct {

testlib/resources.go

@@ -12,10 +12,10 @@ import (
 
 	"github.com/pkg/errors"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/shared/filestore"
-	"github.com/mattermost/mattermost-server/v5/utils"
-	"github.com/mattermost/mattermost-server/v5/utils/fileutils"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/filestore"
+	"github.com/mattermost/mattermost-server/v6/utils"
+	"github.com/mattermost/mattermost-server/v6/utils/fileutils"
 )
 
 const (

testlib/store.go

@@ -7,10 +7,10 @@ import (
 	"net/http"
 	"strconv"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/plugin/plugintest/mock"
-	"github.com/mattermost/mattermost-server/v5/store"
-	"github.com/mattermost/mattermost-server/v5/store/storetest/mocks"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/plugin/plugintest/mock"
+	"github.com/mattermost/mattermost-server/v6/store"
+	"github.com/mattermost/mattermost-server/v6/store/storetest/mocks"
 )
 
 type TestStore struct {