Add latest tag warning

What does this MR do?

Adds a warning to not use the :latest tag when specifying a scanner image to use via the CS_ANALYZER_IMAGE variable.

This is due to the :latest tag being used by versions 4, 5 and 6 of the images and therefore not consistently mapping to the latest version 6 image.

Although the existing documentation is specific regarding the values that should be used for CS_ANALYZER_IMAGE it is not unreasonable for a customer to assume that :latest will map to the latest v6 image, and this is not necessarily the case.

I haven't added an explanation for the warning because it is a bit complicated to explain and the advice is clear on its own.

This arose in a customer support ticket where they were using :latest successfully for a time until an image publishing pipeline fault caused :latest to change from pointing to the v6 image to the v4 image, resulting in unexpected scan job errors, and it was suggested in the related issue below to add a specific warning to the docs.

Related issues

#436677 (comment 1713376198)

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