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
```

manualtesting/manual_testing.go

@@ -12,14 +12,14 @@ import (
 	"strconv"
 	"time"
 
-	"github.com/mattermost/mattermost-server/v5/api4"
-	"github.com/mattermost/mattermost-server/v5/app"
-	"github.com/mattermost/mattermost-server/v5/app/slashcommands"
-	"github.com/mattermost/mattermost-server/v5/model"
-	"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/v5/web"
+	"github.com/mattermost/mattermost-server/v6/api4"
+	"github.com/mattermost/mattermost-server/v6/app"
+	"github.com/mattermost/mattermost-server/v6/app/slashcommands"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/store"
+	"github.com/mattermost/mattermost-server/v6/utils"
+	"github.com/mattermost/mattermost-server/v6/web"
 )
 
 // TestEnvironment is a helper struct used for tests in manualtesting.

manualtesting/test_autolink.go

@@ -6,8 +6,8 @@ package manualtesting
 import (
 	"net/http"
 
-	"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"
 )
 
 const linkPostText = `