Fix snowplow tracking event error for new user invite page

What does this MR do?

References: #241727 (closed)

Introduced by !39628 (merged)

The value property on Snowplow only accepts Numbers or nil values. This caused an issue for the invitation event where we're using a string for value. We're fixing the problem by using the label property instead which accepts Strings as well.

See snowplow docs for details on the types: https://github.com/snowplow/snowplow/wiki/snowplow-tracker-protocol#39-custom-structured-event-tracking

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