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
```

app/context.go

@@ -6,9 +6,9 @@ package app
 import (
 	"context"
 
-	"github.com/mattermost/mattermost-server/v5/app/request"
-	"github.com/mattermost/mattermost-server/v5/plugin"
-	"github.com/mattermost/mattermost-server/v5/store/sqlstore"
+	"github.com/mattermost/mattermost-server/v6/app/request"
+	"github.com/mattermost/mattermost-server/v6/plugin"
+	"github.com/mattermost/mattermost-server/v6/store/sqlstore"
 )
 
 // WithMaster adds the context value that master DB should be selected for this request.