Transit Gateway: Increase acceptance test concurrency

[ewbankkit]

Transit Gateway acceptance tests were serialized a while ago in order to prevent quota exhaustion.
The serialization limits us to one aws_transit_gateway resource acceptance test and one aws_transit_gateway data source acceptance test at a time. This leads to long acceptance test runs in CI for these resources and occasional timeouts.
We should be able to increase the concurrency (to say 5) for each of these using the internal/experimental/sync functionality.

Relates #35769.
Relates #35747.

[github-actions]

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[github-actions]

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[github-actions]

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