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_test.go

@@ -0,0 +1,57 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSlackCompatibleBool_UnmarshalJSON_True(t *testing.T) {
+	cases := []struct {
+		name    string
+		payload string
+	}{
+		{name: "literal", payload: `{"title": "Foo", "value": "Bar", "short": true}`},
+		{name: "stringLower", payload: `{"title": "Foo", "value": "Bar", "short": "true"}`},
+		{name: "stringMixed", payload: `{"title": "Foo", "value": "Bar", "short": "True"}`},
+		{name: "stringUpper", payload: `{"title": "Foo", "value": "Bar", "short": "TRUE"}`},
+	}
+
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			field := &SlackAttachmentField{}
+
+			err := json.Unmarshal([]byte(tt.payload), field)
+
+			require.NoError(t, err)
+			require.True(t, bool(field.Short))
+		})
+	}
+}
+
+func TestSlackCompatibleBool_UnmarshalJSON_False(t *testing.T) {
+	cases := []struct {
+		name    string
+		payload string
+	}{
+		{name: "literal", payload: `{"title": "Foo", "value": "Bar", "short": false}`},
+		{name: "stringLower", payload: `{"title": "Foo", "value": "Bar", "short": "false"}`},
+		{name: "stringMixed", payload: `{"title": "Foo", "value": "Bar", "short": "False"}`},
+		{name: "stringUpper", payload: `{"title": "Foo", "value": "Bar", "short": "FALSE"}`},
+	}
+
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			field := &SlackAttachmentField{}
+
+			err := json.Unmarshal([]byte(tt.payload), field)
+
+			require.NoError(t, err)
+			require.False(t, bool(field.Short))
+		})
+	}
+}