MM-60797: Fix errcheck issues in support_packet_test.go (#30607)

Fixes https://github.com/mattermost/mattermost/issues/29102

- Fix error handling in fileutils.FindDir() call by properly checking the returned boolean value
- Add proper error handling for SetPhase2PermissionsMigrationStatus calls
- Remove channels/app/support_packet_test.go from the errcheck exception list in .golangci.yml

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

server/.golangci.yml

@@ -110,7 +110,6 @@ issues:
         channels/app/slashcommands/auto_environment.go|\
         channels/app/slashcommands/command_test.go|\
         channels/app/slashcommands/helper_test.go|\
-        channels/app/support_packet_test.go|\
         channels/app/team.go|\
         channels/app/team_test.go|\
         channels/app/upload.go|\