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
```

web/web_test.go

@@ -16,15 +16,15 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/app"
-	"github.com/mattermost/mattermost-server/v5/app/request"
-	"github.com/mattermost/mattermost-server/v5/config"
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/plugin"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
-	"github.com/mattermost/mattermost-server/v5/store/localcachelayer"
-	"github.com/mattermost/mattermost-server/v5/store/storetest/mocks"
-	"github.com/mattermost/mattermost-server/v5/utils"
+	"github.com/mattermost/mattermost-server/v6/app"
+	"github.com/mattermost/mattermost-server/v6/app/request"
+	"github.com/mattermost/mattermost-server/v6/config"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/plugin"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/store/localcachelayer"
+	"github.com/mattermost/mattermost-server/v6/store/storetest/mocks"
+	"github.com/mattermost/mattermost-server/v6/utils"
 )
 
 var ApiClient *model.Client4
@@ -192,7 +192,7 @@ func TestStaticFilesRequest(t *testing.T) {
 	package main
 
 	import (
-		"github.com/mattermost/mattermost-server/v5/plugin"
+		"github.com/mattermost/mattermost-server/v6/plugin"
 	)
 
 	type MyPlugin struct {
@@ -279,7 +279,7 @@ func TestPublicFilesRequest(t *testing.T) {
 	package main
 
 	import (
-		"github.com/mattermost/mattermost-server/v5/plugin"
+		"github.com/mattermost/mattermost-server/v6/plugin"
 	)
 
 	type MyPlugin struct {