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/model/plugin_key_value_test.go

@@ -0,0 +1,25 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPluginKeyIsValid(t *testing.T) {
+	kv := PluginKeyValue{PluginId: "someid", Key: "somekey", Value: []byte("somevalue")}
+	assert.Nil(t, kv.IsValid())
+
+	kv.PluginId = ""
+	assert.NotNil(t, kv.IsValid())
+
+	kv.PluginId = "someid"
+	kv.Key = ""
+	assert.NotNil(t, kv.IsValid())
+
+	kv.Key = "this is an extremely long, long, long, long, long, long, long, long, long, long, long, long, long key and should be invalid and this is being verified in this test"
+	assert.NotNil(t, kv.IsValid())
+}