docs: update env var (holder) for images based on Server version

[kelvintaywl]

Description

I changed the SERVER_4_x_IMAGE_LIST env var used in our example scripts to reflect the Server 4.x version correctly.

Reasons

Server 4.1 and 4.2 were consistent;
From Server 4.3 onwards, this wasn't changed. Hence, this PR to clean up for clarity.

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