Marking PayFlow, PayPal Standard, and Braintree as deprecated

[dparker1005]

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Changes proposed in this Pull Request:

Sites already using these gateways can continue using them, but they will not be shown as options on new sites.

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Changelog entry

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[ideadude]

Looks good. Ready to merge.

FYI it seems we were loading the Braintree class early so the dependency checks would run even if not being used, which is okay to skip now that we are deprecating.