multi: rename experimental endorsement signal to accountable

[elnosh]

This PR renames bLIP4's experimental endorsement signal to accountable to match the latest proposal in https://github.com/lightning/blips/pull/67

[gemini-code-assist[bot]]

Summary of Changes

Hello @elnosh, I'm Gemini Code Assist[^1]! 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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[lightninglabs-deploy]

@elnosh, remember to re-request review from reviewers when ready

[Roasbeef]

Approved the CI run.