use YAML.unsafe_load to support Ruby 3.1/Psych 4

[gyugyu]

Since YAML.load has been changed to an alias of Psych.safe_load from Ruby 3.1/Psych 4.0, using Date, Time, etc. cause raise owing to prohibiting of class loading at frontmatter.
ruby/psych#487
To avoid this problem, use YAML.unsafe_load if it is definded (YAML.load is renamed to YAML.unsafe_load).
I think this is better implementation because

• This frontmatter comes from developer's input, so it may be treated as safe input.
• This modification will not break previous behavior of Middleman.

but you can choose passing permitted_classes option to YAML.load to fix vulnerability.

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[markets]

Hi @gyugyu 👋🏼

That seems fine! But in order to get a green ✅ build, could you please rebase middleman:4.x? Thanks!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.