fix: tmux unknown cmd: attach-client

[SteveLauC]

What does this PR do

Based on this comment, fixes #928.

Standards checklist

• [x] The PR title is descriptive.
• [x] I have read CONTRIBUTING.md
• [ ] Optional: I have tested the code myself

For new steps

• [ ] Optional: Topgrade skips this step where needed
• [ ] Optional: The --dry-run option works with this step
• [ ] Optional: The --yes option works with this step if it is supported by the underlying command

If you developed a feature or a bug fix for someone else and you do not have the means to test it, please tag this person here.

[SteveLauC]

Here is a build of Topgrade with this patch included, cc @rabin-io @lucaspar, could you guys please give it a test?

[rabin-io]

@SteveLauC looks good from my end, works with my current config (as above).

[SteveLauC]

Thanks for testing it!