v8.0 module release (#22975)

https://mattermost.atlassian.net/browse/MM-52079

```release-note
We upgrade the module version to 8.0. The new module path is github.com/mattermost-server/server/v8.
```


Co-authored-by: Doug Lauder <wiggin77@warpmail.net>

server/plugin/example_hello_world_test.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package plugin_test
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/mattermost/mattermost-server/server/v8/plugin"
+)
+
+// HelloWorldPlugin implements the interface expected by the Mattermost server to communicate
+// between the server and plugin processes.
+type HelloWorldPlugin struct {
+	plugin.MattermostPlugin
+}
+
+// ServeHTTP demonstrates a plugin that handles HTTP requests by greeting the world.
+func (p *HelloWorldPlugin) ServeHTTP(c *plugin.Context, w http.ResponseWriter, r *http.Request) {
+	fmt.Fprint(w, "Hello, world!")
+}
+
+// This example demonstrates a plugin that handles HTTP requests which respond by greeting the
+// world.
+func Example_helloWorld() {
+	plugin.ClientMain(&HelloWorldPlugin{})
+}