Update actions/checkout in consistency check job to v6

[krzema12]

See

github-workflows-kt/github-workflows-kt/src/main/kotlin/io/github/typesafegithub/workflows/yaml/ConsistencyCheckJob.kt

Line 49 in 0ff9e49

actionVersion = "v4",

There's already v5: https://github.com/actions/checkout/releases/tag/v5.0.0

[krzema12]

There's v6 already.

Actually,, there's an idea for a more maintainable approach: #2157.