Update documentation with SSH requirements and Ubuntu 22 support

[devin-ai-integration]

Update documentation with SSH requirements and Ubuntu 22 support

Summary

This PR adds explicit documentation for two key requirements that were previously implicit in the Brev Cloud SDK:

1. SSH Access Requirements: Added comprehensive documentation specifying that Brev depends on SSH server functionality and SSH key-based authentication for instance access
2. Platform Support: Explicitly documented that the SDK currently supports Ubuntu 22 only

The changes include updates to the main security documentation and all three provider-specific security files (Lambda Labs, Nebius, FluidStack) to ensure consistency across the SDK documentation.

Review & Testing Checklist for Human

• Verify SSH server support claims: Confirm that Lambda Labs, Nebius, and FluidStack actually support SSH server pre-installation and SSH key injection as documented
• Validate Ubuntu 22 requirement: Check if "Ubuntu 22 only" is accurate or if other Ubuntu versions/OSes should be supported
• Review SSH security integration: Ensure SSH access requirements align with each provider's firewall rules and security group configurations
• Test SSH implementation: Verify that actual SDK implementations support the documented SSH key injection and server access methods

Recommended Test Plan: Create test instances with each provider to verify SSH server is running, SSH key injection works during provisioning, and access works within documented security models.

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    MainSec["docs/SECURITY.md<br/>Main Security Requirements"]:::major-edit
    ReadMe["README.md<br/>Project Overview"]:::major-edit
    
    LambdaSec["internal/lambdalabs/SECURITY.md<br/>Lambda Labs Provider"]:::major-edit
    NebiusSec["internal/nebius/SECURITY.md<br/>Nebius Provider"]:::major-edit
    FluidSec["internal/fluidstack/v1/SECURITY.md<br/>FluidStack Provider"]:::major-edit
    
    MainSec --> LambdaSec
    MainSec --> NebiusSec
    MainSec --> FluidSec
    
    ReadMe -.-> MainSec
    
    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit
        L3[Context/No Edit]:::context
    end

    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

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Notes

• All changes are documentation-only with no code modifications
• SSH requirements were added consistently across all provider documentation
• Ubuntu 22 support specification was added to main README
• Documentation follows existing security model patterns (deny inbound, allow outbound)
• Changes maintain consistency with each provider's specific security implementation details

Session Info: https://app.devin.ai/sessions/5c677003f37d40f3b4a28597f8c3687a
Requested by: Alec Fong (@theFong)

[devin-ai-integration]

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