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/doc.go

@@ -0,0 +1,9 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+// The plugin package is used by Mattermost server plugins written in go. It also enables the
+// Mattermost server to manage and interact with the running plugin environment.
+//
+// Note that this package exports a large number of types prefixed with Z_. These are public only
+// to allow their use with Hashicorp's go-plugin (and net/rpc). Do not use these directly.
+package plugin