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/plugintest/example_hello_user_test.go

@@ -0,0 +1,58 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package plugintest_test
+
+import (
+	"fmt"
+	io "io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mattermost/mattermost-server/server/v8/model"
+	"github.com/mattermost/mattermost-server/server/v8/plugin"
+	"github.com/mattermost/mattermost-server/server/v8/plugin/plugintest"
+)
+
+type HelloUserPlugin struct {
+	plugin.MattermostPlugin
+}
+
+func (p *HelloUserPlugin) ServeHTTP(context *plugin.Context, w http.ResponseWriter, r *http.Request) {
+	userID := r.Header.Get("Mattermost-User-Id")
+	user, err := p.API.GetUser(userID)
+	if err != nil {
+		w.WriteHeader(http.StatusBadRequest)
+		p.API.LogError(err.Error())
+		return
+	}
+
+	fmt.Fprintf(w, "Welcome back, %s!", user.Username)
+}
+
+func Example() {
+	t := &testing.T{}
+	user := &model.User{
+		Id:       model.NewId(),
+		Username: "billybob",
+	}
+
+	api := &plugintest.API{}
+	api.On("GetUser", user.Id).Return(user, nil)
+	defer api.AssertExpectations(t)
+
+	p := &HelloUserPlugin{}
+	p.SetAPI(api)
+
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest("GET", "/", nil)
+	r.Header.Add("Mattermost-User-Id", user.Id)
+	p.ServeHTTP(&plugin.Context{}, w, r)
+	body, err := io.ReadAll(w.Result().Body)
+	require.NoError(t, err)
+	assert.Equal(t, "Welcome back, billybob!", string(body))
+}