refactor: make linked list iterator lifetimes explicit

[mevinagrise]

This PR makes the return lifetimes of LinkedList::iter and LinkedList::iter_mut explicit.

Specific changes include:

• Updated LinkedList::iter to return Iter<'_>.
• Updated LinkedList::iter_mut to return IterMut<'_>.
• Kept behavior unchanged while removing the current mismatched_lifetime_syntaxes warnings on newer toolchains.

Why this is useful:

• This is a minimal code health improvement.
• It keeps the API behavior exactly the same.
• It reduces warning noise during cargo test and cargo test --doc.

Testing:

• Ran cargo test.
• Ran cargo test --doc.

[mevinagrise]

Just noticed the issue is already fixed in the new release. Closing this PR now. Thanks!