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/searchtest/channel_layer.go

@@ -8,8 +8,8 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	"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"
 )
 
 var searchChannelStoreTests = []searchTest{

store/searchtest/file_info_layer.go

@@ -9,8 +9,8 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	"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"
 )
 
 var searchFileInfoStoreTests = []searchTest{

store/searchtest/helper.go

@@ -11,8 +11,8 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"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"
 )
 
 type SearchTestHelper struct {

store/searchtest/post_layer.go

@@ -9,8 +9,8 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	"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"
 )
 
 var searchPostStoreTests = []searchTest{

store/searchtest/testlib.go

@@ -6,8 +6,8 @@ package searchtest
 import (
 	"testing"
 
-	"github.com/mattermost/mattermost-server/v5/store"
-	"github.com/mattermost/mattermost-server/v5/utils"
+	"github.com/mattermost/mattermost-server/v6/store"
+	"github.com/mattermost/mattermost-server/v6/utils"
 )
 
 const (

store/searchtest/user_layer.go

@@ -8,8 +8,8 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	"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"
 )
 
 var searchUserStoreTests = []searchTest{