Fix deprecation warning

[deining]

When previewing the theme, deprecation warnings are shown with latest released hugo version 0.157.0:

INFO  deprecated: .Site.Data was deprecated in Hugo v0.156.0 and will be removed in a future release.
Use hugo.Data instead.
INFO  deprecated: .Site.Sites and .Page.Sites was deprecated in Hugo v0.156.0 and will be removed in a future release.
Use hugo.Sites instead.

This PR fixes that issue.

Note:: This PR raises the minimum required hugo version to 0.156.0.

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[imfing]

Hey, thanks for the pull request! Just wanted to mention that this would mean everyone would need to upgrade to the latest version, which is a pretty big change.

[deining]

Hey, thanks for the pull request!

You are welcome!

Just wanted to mention that this would mean everyone would need to upgrade to the latest version

You are right. I just added this to the PR description above.

which is a pretty big change.

Feel free to merge this PR whenevr you think it's appropriate.