update Magnus library in Rust extension gem template

[ggmichaelgo]

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.

The newer version of Magnus uses the rb-sys' new stable API for RArray and RString. This update will increase stability of Rust gems to be used with different versions of Ruby.
Also, the newer version fixes the compile error in RString#bytes feature.

Thank you @matsadler @ianks and @gmalette for these update on Magnus!

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

[welcome]

Thanks for opening a pull request and helping make RubyGems and Bundler better! Someone from the RubyGems team will take a look at your pull request shortly and leave any feedback. Please make sure that your pull request has tests for any changes or added functionality.

We use GitHub Actions to test and make sure your change works functionally and uses acceptable conventions, you can review the current progress of GitHub Actions in the PR status window below.

If you have any questions or concerns that you wish to ask, feel free to leave a comment in this PR or join our #rubygems or #bundler channel on Slack.

For more information about contributing to the RubyGems project feel free to review our CONTRIBUTING guide