Remove deprecated environment_tier param from DORA API

environment_tier param should be deprecated in favour of environment_tiers param to avoid duplication and confusion.

Timeline

Please add links to the relevant merge requests.

As soon as possible, but no later than the third milestone preceding the major release (for example, given the following release schedule: 14.8, 14.9, 14.10, 15.0 – 14.8 is the third milestone preceding the major release):
- A deprecation entry has been created so the deprecation will appear in release posts and on the general deprecation page.
- Documentation has been updated to mark the feature as deprecated.
On or before the major milestone: A removal entry has been created so the removal will appear on the removals by milestones page and be announced in the release post.
On the major milestone:
- The deprecated item has been removed.
- If the removal of the deprecated item is a breaking change, the merge request is labeled breaking change.

Mentions

Your stage's stable counterparts have been @mentioned on this issue. For example, Customer Support, Customer Success (Technical Account Manager), Product Marketing Manager.
- To see who the stable counterparts are for a product team visit product categories
  - If there is no stable counterpart listed for Sales/CS please mention @timtams
  - If there is no stable counterpart listed for Support please mention @gitlab-com/support/managers
  - If there is no stable counterpart listed for Marketing please mention @cfoster3
Your GPM has been @mentioned so that they are aware of planned deprecations. The goal is to have reviews happen at least two releases before the final removal of the feature or introduction of a breaking change.

Edited Apr 27, 2023 by Adam Hegyi