Fix code style and enforce with GitHub Actions test

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Resolves: No issue in tracker

Strictly enforcing the code style will make it easier to cherry-pick commits from the 3.x branch.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• [N/A] I made sure the commit message title starts with "fix #424242:" if there is a related issue
• [N/A] I created the test (mtest, vtest, script test) to verify the changes I made

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Sample output for when the code style test fails:

Scrolling down under "Test coding style" reveals the changes required to pass the test:

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Now that this has been merged, the code style test is enforced for all future PRs. However, pre-existing PRs will not been tested for coding style unless the GitHub Action for MacOS is triggered again (e.g. by pushing another commit to the PR branch).

If you see this under a PR, it has not been tested for coding style:

PRs that have been tested will be labelled like this: (notice that the MacOS Action name begins with "codestyle")

Regardless of test status, coding style needs to be fixed before PRs are merged otherwise the test will start to fail for everybody.