Prepare index for environments on project_id and id

What does this MR do and why?

This MR prepares an index for the environments table on the project_id and id columns (index_environments_on_project_id_and_id). In the next step, we'll use add_concurrent_index to complete it. (https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#create-indexes-asynchronously)

The main reason for this index is to increase the performance of the environments list endpoint. More info: #385767 (comment 1230613048)

Related to #385767 (closed)

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