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/checker/main.go

@@ -0,0 +1,71 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"sort"
+	"strings"
+)
+
+const pluginPackagePath = "github.com/mattermost/mattermost-server/server/v8/plugin"
+
+type result struct {
+	Warnings []string
+	Errors   []string
+}
+
+type checkFn func(pkgPath string) (result, error)
+
+var checks = []checkFn{
+	checkAPIVersionComments,
+}
+
+func main() {
+	var res result
+	for _, check := range checks {
+		res = runCheck(res, check)
+	}
+
+	var msgs []string
+	msgs = append(msgs, res.Errors...)
+	msgs = append(msgs, res.Warnings...)
+	sort.Strings(msgs)
+
+	if len(msgs) > 0 {
+		fmt.Fprintln(os.Stderr, "#", pluginPackagePath)
+		fmt.Fprintln(os.Stderr, strings.Join(msgs, "\n"))
+	}
+
+	if len(res.Errors) > 0 {
+		os.Exit(1)
+	}
+}
+
+func runCheck(prev result, fn checkFn) result {
+	res, err := fn(pluginPackagePath)
+	if err != nil {
+		prev.Errors = append(prev.Errors, err.Error())
+		return prev
+	}
+
+	if len(res.Warnings) > 0 {
+		prev.Warnings = append(prev.Warnings, mapWarnings(res.Warnings)...)
+	}
+
+	if len(res.Errors) > 0 {
+		prev.Errors = append(prev.Errors, res.Errors...)
+	}
+
+	return prev
+}
+
+func mapWarnings(ss []string) []string {
+	var out []string
+	for _, s := range ss {
+		out = append(out, "[warn] "+s)
+	}
+	return out
+}