T3a instances and max ENIs

Looks very similar to issue #1542.

Currently trying to deploy two tasks to a `t3a.small` instance using `awsvpc` networking. According to https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html, a `t3a.small` instance should have a maximum of 3 ENIs and can therefore host 2 tasks using the `awsvpc` networking mode.

Currently when deploying only one task successfully deploys while the other fails with `RESOURCE:ENI`. The work around at the moment is to use a `t3.small` instance. When I switch to that type I am able to deploy the two tasks successfully.

Does the fix for the original issue with `t3` (#1542) instances need to be applied for `t3a` instance?

Begin with issue #1542 and the linked AWS ENI documentation; reproduce the two-task deployment on a t3a.small using awsvpc and compare it with t3.small. Done means determining whether the t3a ENI behavior is incorrect and recording a confirmed fix or limitation for the reported RESOURCE:ENI failure.

Written by the indexing model from the issue text.