Recommending using Stripe restricted keys

[dparker1005]

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Changes proposed in this Pull Request:

Stripe is beginning to require authentication with restricted keys or Stripe connect. This PR hides some "legacy key" errors when using restricted keys and removes "legacy key" wording from some places.

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Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

[ideadude]

Minor wording tweak