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/slack_compatibility.go

@@ -0,0 +1,30 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import (
+	"fmt"
+	"strings"
+)
+
+// SlackCompatibleBool is an alias for bool that implements json.Unmarshaler
+type SlackCompatibleBool bool
+
+// UnmarshalJSON implements json.Unmarshaler
+//
+// Slack allows bool values to be represented as strings ("true"/"false") or
+// literals (true/false). To maintain compatibility, we define an Unmarshaler
+// that supports both.
+func (b *SlackCompatibleBool) UnmarshalJSON(data []byte) error {
+	value := strings.ToLower(string(data))
+	if value == "true" || value == `"true"` {
+		*b = true
+	} else if value == "false" || value == `"false"` {
+		*b = false
+	} else {
+		return fmt.Errorf("unmarshal: unable to convert %s to bool", data)
+	}
+
+	return nil
+}