Clarify Defender integration docs for API keys

[ericglau]

On https://docs.openzeppelin.com/upgrades-plugins/1.x/defender-deploy and https://docs.openzeppelin.com/upgrades-plugins/1.x/foundry-defender, clarify that the API key and secret referenced in the following statement is for the Production or Test Deploy Environment that was configured in Defender:

Set the following environment variables in your .env file at your project root, using your Team API key and secret from OpenZeppelin Defender:

Consider linking to https://docs.openzeppelin.com/defender/v2/tutorial/deploy#environment_setup

See https://forum.openzeppelin.com/t/add-support-to-create-proposals-through-the-openzeppelin-defender-within-a-foundry-project/38754/8