Bump actions/checkout to v4 in bundler gem template

[ytkg]

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

Running bundler gem --ci=github generated a workflow with an out of date actions/checkout step

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

Update the version of actions/checkout in the generated workflow template

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

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[hsbt]

👍