Support all DX connection speeds

[ewbankkit]

Fixes #6043.
Acceptance tests:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSDxConnection_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -run=TestAccAWSDxConnection_basic -timeout 120m
=== RUN   TestAccAWSDxConnection_basic
--- PASS: TestAccAWSDxConnection_basic (41.97s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	59.373s
$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSDxLag_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -run=TestAccAWSDxLag_basic -timeout 120m
=== RUN   TestAccAWSDxLag_basic
--- PASS: TestAccAWSDxLag_basic (118.48s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	119.199s

[bflad]

Whoops meant to hit "approve" before merge 😄 LGTM!

[ghost]

This has been released in version 1.39.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "aws" {
	version = "~> 1.39.0"
}
# ... other configuration ...

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!