multi: rename experimental endorsement signal to accountable

[elnosh]

This PR renames bLIP4's experimental endorsement signal to accountable to match the latest proposal in lightning/blips#67

[gemini-code-assist]

Summary of Changes

Hello @elnosh, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request implements a comprehensive renaming of the experimental endorsement signal to accountable. This change is crucial for aligning the codebase with the latest BLIP-0004 proposal, ensuring that the terminology used for this experimental feature is consistent and up-to-date across all relevant components, including feature management, HTLC forwarding logic, configuration, and testing infrastructure.

Highlights

• Terminology Update: The experimental endorsement signal has been renamed to accountable across the entire codebase to align with the latest BLIP-0004 proposal.
• Configuration Alignment: Configuration options, feature bits, and command-line flags related to this experimental signal have been updated to reflect the new accountable terminology.
• Codebase Consistency: All relevant function names, variable names, constants, and test cases have been refactored to use the accountable term, ensuring consistency throughout the project.

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[gemini-code-assist]

Code Review

This pull request correctly renames the experimental endorsement feature to accountable to align with the latest bLIP-4 proposal. The changes are applied consistently across the entire codebase, including configuration options, feature flags, internal logic, and tests. I've also noted the removal of the time-based experiment logic for this feature, which is a correct interpretation of the updated proposal. The code quality is high, and I have no further comments. The changes look good to merge.