Remove deprecated environment_tier param

DO NOT MERGE THIS MR

We need to wait until the last week of the milestone.

What does this MR do and why?

This change removes the deprecated environment_tier parameter from the GraphQL DORA API.

• The deprecation notice was merged a few months ago: !108028 (merged)
• Removal entry: !118892 (merged)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #365939 (closed)