Font collection: add terms and conditions property

[matiasbenedetto]

What?

Adds a terms_and_conditions property in the font collections.

Why?

Trac ticket: https://core.trac.wordpress.org/ticket/60593

Note:

Disclaimer: This PR includes the changes added in #6130. That PR includes relevant changes so I started this branch from it for this PR. The commit history of this PR will be cleaned-up when #6130 is merged and the updated trunk is merged in this branch.

[github-actions]

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Core Committers: Use this line as a base for the props when committing in SVN:

Props swissspidy, mmaattiiaass.

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[github-actions]

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