Remove Cargo.lock from being default ignore as per updated guidance

[cdbrkfxrpt]

New or update

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New

• [ ] Template - New .gitignore template
• [ ] Composition - Template made from smaller templates
• [ ] Inheritance - Template similar to an existing template
• [ ] Patch - Template extending functionality of existing template

Update

• [x] Template - Update existing .gitignore template

Details

On 29th August 2023, there has been a change in guidance on committing lockfiles.

The previous guidance was to commit the Cargo.lock file for executables, and to ignore it for libraries (as was the behaviour of cargo when creating new projects via cargo new versus cargo new --lib, and the behaviour of the template.

The new guidance is to "do what's best for your project" and to commit Cargo.lock for all new repos as a starting point. Since Rust nightly-2023-08-24, this is the behaviour of cargo and this PR adjusts the template accordingly.