Bump rtl to 0.15.6 and boltz client to 2.8.9

[kilrau]

No description provided.

[github-actions]

🎉   Linting finished with no errors or warnings   🎉

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Linting Results

Severity	File	Description
ℹ️	core-lightning-rtl/docker-compose.yml	Potentially using unsafe user in service "web": The default container user "root" can lead to security vulnerabilities. If you are using the root user, please try to specify a different user (e.g. "1000:1000") in the compose file or try to set the UID/PUID and GID/PGID environment variables to 1000.
ℹ️	core-lightning-rtl/docker-compose.yml	Potentially using unsafe user in service "boltz": The default container user "root" can lead to security vulnerabilities. If you are using the root user, please try to specify a different user (e.g. "1000:1000") in the compose file or try to set the UID/PUID and GID/PGID environment variables to 1000.

Legend

Symbol	Description
❌	Error: This must be resolved before this PR can be merged.
⚠️	Warning: This is highly encouraged to be resolved, but is not strictly mandatory.
ℹ️	Info: This is just for your information.

[nmfretz]

Thanks for these 2 updates @kilrau! Really appreciate you keeping these up-to-date in our app store 🤝

Reviewed and tested.