Remove cargo's audit allowlist that is no longer needed

[isimluk]

The allowlist item has been introduced by #2928 & #2955. The item is no longer needed after chrono has been fixed on 0.4.20 in #3264.

[isimluk]

Closing for now as I have realized that 0.4.19 version of chrono is still in use.

diesel/Cargo.toml:chrono = { version = "0.4.20", optional = true, default-features = false, features = ["clock", "std"] }
diesel_cli/Cargo.toml:chrono = { version = "0.4.19", default-features = false, features = ["clock", "std"] }
diesel_tests/Cargo.toml:chrono = { version = "0.4.19", default-features = false, features = ["clock", "std"] }
examples/mysql/all_about_inserts/Cargo.toml:chrono = { version = "0.4.19", default-features = false, features = ["clock", "std"] }
examples/postgres/advanced-blog-cli/Cargo.toml:chrono = { version = "0.4.19", default-features = false, features = ["clock", "std"] }
examples/sqlite/all_about_inserts/Cargo.toml:chrono = { version = "0.4.19", default-features = false, features = ["clock", "std"] }

Sorry for the noise.

[weiznich]

@isimluk I think it should be fine to just update the chrono version in all that crates. I would accept a PR for that.

[isimluk]

Thanks for the confirmation, I am investigating that. I also just realized that 0.4.20 gets picked up automatically, even in cases where 0.4.19 gets spelled out.