Clean up dead code leftover from the Rust webhook server used for mocking requests

[emresurmeli]

The Rust-based mock webhook server was replaced with a lightweight JavaScript-based mock webhook logger in PR #624 (commit 4da98f7) on October 18, 2024.

The Rust webhook server files still exist in the webhook-servers/ directory, but are now dead code:

• webhook-servers/src/api.rs
• webhook-servers/src/main.rs
• webhook-servers/src/types.rs
• webhook-servers/Cargo.toml
• webhook-servers/Cargo.lock
• webhook-servers/.gitignore
• Docker/Dockerfile.mock-webhook-server
• .prettierignore - Remove line 7: services/account/webhook-servers/target (outdated path, redundant with line 14)