Create Index on Environments project_id, state, environment_type

What does this MR do?

Replace the index on the environments table on project_id, state with one on project_id, state, environment_type.

This is from discussion in !19775 (merged).

See !19775 (comment 242656843)

Issue: #3713 (closed)

This index is meant to optimize the following query (and similar with different id's):

SELECT environments.* FROM (SELECT environments.*, rank() OVER (PARTITION BY project_id ORDER BY id DESC) FROM environments WHERE environments.environment_type IS NULL AND (environments.state IN ('available'::text))) environments WHERE (rank <= 3) AND environments.project_id IN (278964, 7764);

Produced by preloading this association:

has_many :environments_for_dashboard, -> { from(with_rank.unfoldered.available, :environments).where('rank <= 3') }, class_name: 'Environment'

Found in the Environment model: !19775 (diffs)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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