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/public/plugin/plugintest/api.go

@@ -10,7 +10,7 @@ import (
 
 	mock "github.com/stretchr/testify/mock"
 
-	model "github.com/mattermost/mattermost-server/server/public/model"
+	model "github.com/mattermost/mattermost/server/public/model"
 )
 
 // API is an autogenerated mock type for the API type

server/public/plugin/plugintest/doc.go

@@ -7,5 +7,5 @@
 // https://godoc.org/github.com/stretchr/testify/mock
 //
 // If you need to import the mock package, you can import it with
-// "github.com/mattermost/mattermost-server/server/public/plugin/plugintest/mock".
+// "github.com/mattermost/mattermost/server/public/plugin/plugintest/mock".
 package plugintest

server/public/plugin/plugintest/driver.go

@@ -9,7 +9,7 @@ import (
 
 	mock "github.com/stretchr/testify/mock"
 
-	plugin "github.com/mattermost/mattermost-server/server/public/plugin"
+	plugin "github.com/mattermost/mattermost/server/public/plugin"
 )
 
 // Driver is an autogenerated mock type for the Driver type

server/public/plugin/plugintest/example_hello_user_test.go

@@ -13,9 +13,9 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/server/public/model"
-	"github.com/mattermost/mattermost-server/server/public/plugin"
-	"github.com/mattermost/mattermost-server/server/public/plugin/plugintest"
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/plugin"
+	"github.com/mattermost/mattermost/server/public/plugin/plugintest"
 )
 
 type HelloUserPlugin struct {

server/public/plugin/plugintest/hooks.go

@@ -10,9 +10,9 @@ import (
 
 	mock "github.com/stretchr/testify/mock"
 
-	model "github.com/mattermost/mattermost-server/server/public/model"
+	model "github.com/mattermost/mattermost/server/public/model"
 
-	plugin "github.com/mattermost/mattermost-server/server/public/plugin"
+	plugin "github.com/mattermost/mattermost/server/public/plugin"
 )
 
 // Hooks is an autogenerated mock type for the Hooks type