Swap react-native-dotenv for react-native-config

[markmur]

What changes are you making?

react-native-dotenv has been giving us persistent trouble. Sometimes the env vars are available, other times they're not - with no way of predicting when it might break.

This PR swaps out react-native-dotenv for react-native-config which seems to work more consistently.

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PR Checklist

Important

• I've added tests to support my implementation
• I have read and agree with the Contribution Guidelines.
• I have read and agree with the Code of Conduct.
• I've updated the README.
• I've updated any documentation related to these changes.

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Checklist for releasing a new version

• I have bumped the version number in the package.json file.
• I have added a Changelog entry.

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