Replaced outdated security warnings with accurate information about
the pulse-temp-proxy architecture:
- Removed scary 'legacy feature' and 'compromised container' warnings
- Explains secure proxy architecture for containerized deployments
- Notes that SSH keys are stored on Proxmox host (not in container)
- Clarifies container compromise does not expose credentials
- Includes information for both containerized and native installs
- More factual and less alarmist tone
The old message implied temperature monitoring was insecure for
containers, which is no longer true with pulse-temp-proxy.
Related to #528
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