CI: bump actions/checkout from v3/v4 to v6

[tnir]

Why

With v3, we have the following deprecation notices:

The following actions use a deprecated Node.js version and will be forced to run on node20: actions/checkout@v3. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

from https://github.com/collectiveidea/awesome_nested_set/actions/runs/11509088498.

See https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

Node16 has been out of support since September 2023. As a result we have started the deprecation process of Node16 for GitHub Actions. We plan to migrate all actions to run on Node20 by Spring 2024.
Following on from our warning in workflows using Node16 we will start enforcing the use of Node20 rather than Node16 on the 3rd of June.

What

Bump actions/checkout from v3 (and v4) to v6.

[stale]

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