MM-36271: Bump major version to 6.0 (#17973)

https://mattermost.atlassian.net/browse/MM-36271

```release-note
We bump the major version to 6.0
```

services/slackimport/converters.go

@@ -8,7 +8,7 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 func slackConvertTimeStamp(ts string) int64 {

services/slackimport/main_test.go

@@ -8,7 +8,7 @@ import (
 	"os"
 	"testing"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 func TestMain(m *testing.M) {

services/slackimport/parsers.go

@@ -7,8 +7,8 @@ import (
 	"encoding/json"
 	"io"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 func slackParseChannels(data io.Reader, channelType model.ChannelType) ([]slackChannel, error) {

services/slackimport/slackimport.go

@@ -17,11 +17,11 @@ import (
 	"time"
 	"unicode/utf8"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/shared/i18n"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
-	"github.com/mattermost/mattermost-server/v5/store"
-	"github.com/mattermost/mattermost-server/v5/utils"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/i18n"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/store"
+	"github.com/mattermost/mattermost-server/v6/utils"
 )
 
 type slackChannel struct {

services/slackimport/slackimport_test.go

@@ -11,8 +11,8 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/store/storetest/mocks"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/store/storetest/mocks"
 )
 
 func TestSlackConvertTimeStamp(t *testing.T) {