Update GitHub Actions workflow

[uschmidt83]

Update action versions for 'setup-python' and 'cache'.

Set 'fetch-depth: 0' for 'checkout' action to obtain correct document dates when using 'mkdocs-git-revision-date-localized-plugin' (cf. https://github.com/timvink/mkdocs-git-revision-date-localized-plugin/blob/v1.2.2/mkdocs_git_revision_date_localized_plugin/ci.py#L37-L48)

[squidfunk]

Thanks for suggesting. There are two changes in this PR that need discussion. We're happy to update the versions of the GitHub Actions, but we should not add fetch depth. The reason is that not all users are using the git revision date plugin, and adding fetch depth will significantly slow down your build, as it now fetches the entire history. This must be a deliberate decision. Imagine the cost that would add to all projects using Material for MkDocs, not using the history but fetching everything nonetheless. The publishing your site guide aims to be as minimal as possible.

If you remove this change, we're happy to merge your PR.

[uschmidt83]

The reason is that not all users are using the git revision date plugin, and adding fetch depth will significantly slow down your build, as it now fetches the entire history.

Sure. I didn't realize that it would (cumulatively) be a big deal.

I also overlooked the When using build environments box under the Document dates.

[squidfunk]

Thanks! Yeah, it would be a huge waste of resources.