fix: newrelic_notification_channel property should be optional and match what documentation says

[knapo]

Description

Some of newrelic_notification_channel types (e.g. EMAIL) does not require property and it shouldn't be a required value. Also documentation doesn't say it should be required and all possible properties for EMAIL are optional.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My commit message follows conventional commits
• I have performed a self-review of my own code
• New and existing unit tests pass locally with my changes. Go here for instructions on running tests locally.

[CLAassistant]

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