MM-53032: Fix module path after repo rename (#23689)

It was a good decision in hindsight to keep the public module as 0.x
because this would have been a breaking change again.

https://mattermost.atlassian.net/browse/MM-53032
```release-note
Changed the Go module path from github.com/mattermost/mattermost-server/server/v8 to github.com/mattermost/mattermost/server/v8.

For the public facing module, it's path is also changed from github.com/mattermost/mattermost-server/server/public to github.com/mattermost/mattermost/server/public
```

server/channels/utils/testutils/static_config_service.go

@@ -6,7 +6,7 @@ package testutils
 import (
 	"crypto/ecdsa"
 
-	"github.com/mattermost/mattermost-server/server/public/model"
+	"github.com/mattermost/mattermost/server/public/model"
 )
 
 type StaticConfigService struct {

server/channels/utils/testutils/testutils.go

@@ -14,9 +14,9 @@ import (
 	"strconv"
 	"time"
 
-	"github.com/mattermost/mattermost-server/server/v8/channels/utils"
+	"github.com/mattermost/mattermost/server/v8/channels/utils"
 
-	"github.com/mattermost/mattermost-server/server/v8/channels/utils/fileutils"
+	"github.com/mattermost/mattermost/server/v8/channels/utils/fileutils"
 )
 
 func ReadTestFile(name string) ([]byte, error) {