chore: migrate lint configuration to `[workspace.lints]` in Cargo.toml

[josecelano]

Summary

Migrate the ad-hoc lint configuration currently spread across .cargo/config.toml RUSTFLAGS and torrust-linting CLI arguments into a single [workspace.lints] section in Cargo.toml, following the idiomatic Cargo approach.

Background

Lint enforcement is split across three places:

1. .cargo/config.toml RUSTFLAGS — rust-group denials invisible without reading the config
2. torrust-linting clippy runner — only active when the linter tool runs
3. Root [lints.clippy] package section — not propagated to other workspace members; also contains a needless_return = "allow" marked # temp allow this lint whose original reason is unknown

Suggested in PR #1784 review by @da2ce7, referencing the torrust-index configuration as the target state.

Spec

Full implementation spec: docs/issues/open/ (after this issue is created the draft will be moved there)

Tasks (summary)

• T1: Add [workspace.lints.rust] to root Cargo.toml
• T2: Add [workspace.lints.clippy] to root Cargo.toml
• T3: Remove lint entries from RUSTFLAGS in .cargo/config.toml
• T4: Remove root [lints.clippy] package section
• T5: Fix any new lint failures
• T6: Coordinate torrust-linting cleanup (remove redundant -D clippy::X flags)
• T7: Investigate and resolve needless_return = "allow"
• T8: Verify all quality gates pass

Related to #1784