Remove month and year duration constants

[sophiajt]

Description

Some of the humanize logic got mixed into the parsing and duration models, so this removes those but keeps the humanized print. With it, we'll no longer support 1yr or 1month, or any duration above 1wk as the former do not have fixed values.

This should help people create more predictable duration constants.

Tests

Make sure you've done the following:

• Add tests that cover your changes, either in the command examples, the crate/tests folder, or in the /tests folder.
• Try to think about corner cases and various ways how your changes could break. Cover them with tests.
• If adding tests is not possible, please document in the PR body a minimal example with steps on how to reproduce so one can verify your change works.

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --workspace --features=extra -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace --features=extra to check that all the tests pass