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
```

services/imageproxy/atmos_camo_test.go

@@ -13,9 +13,9 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/services/httpservice"
-	"github.com/mattermost/mattermost-server/v5/utils/testutils"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/services/httpservice"
+	"github.com/mattermost/mattermost-server/v6/utils/testutils"
 )
 
 func makeTestAtmosCamoProxy() *ImageProxy {

services/imageproxy/imageproxy.go

@@ -11,10 +11,10 @@ import (
 	"strings"
 	"sync"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/services/configservice"
-	"github.com/mattermost/mattermost-server/v5/services/httpservice"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/services/configservice"
+	"github.com/mattermost/mattermost-server/v6/services/httpservice"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 var ErrNotEnabled = Error{errors.New("imageproxy.ImageProxy: image proxy not enabled")}

services/imageproxy/local.go

@@ -16,8 +16,8 @@ import (
 
 	"willnorris.com/go/imageproxy"
 
-	"github.com/mattermost/mattermost-server/v5/services/httpservice"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/services/httpservice"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 var imageContentTypes = []string{

services/imageproxy/local_test.go

@@ -13,9 +13,9 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/services/httpservice"
-	"github.com/mattermost/mattermost-server/v5/utils/testutils"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/services/httpservice"
+	"github.com/mattermost/mattermost-server/v6/utils/testutils"
 )
 
 func makeTestLocalProxy() *ImageProxy {