Update Magnus version in Rust extension gem template

[matsadler]

What was the end-user or developer problem that led to this PR?

Magnus, the Ruby bindings library used in the template for a Rust extension gem has released a newer version. It would be helpful if the generated gem skeleton used the latest version.

What is your fix for the problem, implemented in this PR?

The version number has been updated in the template Cargo.toml, and a few changes have been made to lib.rs.tt to account for API changes.

As the newer version of Magnus requires a Rust version that will support inline variables in format strings (e.g. format!("{foo}") instead of format!("{}", foo)) I've made that change too.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags