Fix workflow syntax warning and update checkout to Node.js 24

[picatz]

What was changed

Wrap the entire if condition in ${{ }}, and update actions/checkout from v4 to v6.

Why?

To resolve Node.js 20 deprecation and expression syntax warnings:

Node.js 20 actions are deprecated. The following actions are running on Node.js 20 and may not work as expected: actions/checkout@v4.

Workflow syntax warning: .github/workflows/require-codeowners.yml#L35
                                                                                                                                     
.github/workflows/require-codeowners.yml (Line: 35, Col: 9): Conditional expression contains literal text outside replacement tokens. This will cause the expression to always evaluate to truthy. Did you mean to put the entire expression inside ${{ }}?