tests: Remove Whisper from test infrastructure

[MateuszKarlic]

Remove Whisper (aka VeeR-ISS) from test infrastructure.

Whisper configs and scripts to generate them are left untouched.

After restoring Renode as ISS, Whisper became obsolete. We are now using Spike and Renode as main ISS for verification and testing purposes.

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Another change is downloading Renode only once, in a separate workflow and caching the binaries for jobs that use Renode