Draft: Set RUBY_GC_HEAP_INIT_SLOTS to 3M for 1 cluster on git + websockets fleets

What does this MR do?

Step 3 in gitlab-com/gl-infra/production#3680 (closed)

This resizes the initial Ruby heap for all Pumas in a single cluster (us-east1-b) on the git and websockets fleets.

Reviewer Check-list

• Diff jobs reviewed
• Do the changes in the Diffs reflect the desired state of this MR?
• Are there changes shown in the diffs unassociated with this MR? - This may require a rebase or further investigation