Archive open-payments-php-snippets repo

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We currently have two PHP snippet repos:

• https://github.com/interledger/open-payments-php
• https://github.com/interledger/open-payments-php-snippets

I believe we can archive php-snippets; however, the README for this repo has more information than the other.

Please look at the READMEs for both repos and update the open-payments-php file only if needed.

Then we can archive the snippets repo (I have the permission to do this, but if you have it too, go for it).