ci/cd: upgrade actions to v4 to avoid NodeJS warning

[MashyBasker]

This PR upgrades actions/checkout from v2 to v4 in the check-unicode.yml workflow, addressing warnings caused by Node.js 12 deprecation. The upgrade uses Node.js 16, eliminating the warnings.

[ThinkOpenly]

Could you add some helpful content in your commit message?

What I see is for the "check-unicode" CI Action, 2 warnings appear as "Annotations". For example, https://github.com/ThinkOpenly/sail-riscv/actions/runs/10198111590.

The error text is:

The following actions uses node12 which is deprecated and will be forced to run on node16: actions/checkout@v2. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/
The following actions uses Node.js version which is deprecated and will be forced to run on node20: actions/checkout@v2. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

So, please add some content that says that the "check-unicode" CI action is emitting warnings, show the warnings text, explain that it is caused by the version of node.JS used in "v2" and that upgrading from "v2" to "v4" addresses the issue.

[ThinkOpenly]

Sorry I wasn't clear. I think you added some contextual information in the PR description. That is helpful, but could you add the same text into the commit message? Commit messages persist as history within the git repository in the log, so are visible in any clones. We always want to convey the issue seen and the solution to future developers.