r/aws_internet_gateway: Allow available as a pending state during gateway detach

[ewbankkit]

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Closes #21792.

Output from acceptance testing:

% make testacc PKG_NAME=internal/service/ec2 TESTARGS='-run=TestAccEC2InternetGateway_'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/ec2/... -v -count 1 -parallel 20 -run=TestAccEC2InternetGateway_ -timeout 180m
=== RUN   TestAccEC2InternetGateway_basic
=== PAUSE TestAccEC2InternetGateway_basic
=== RUN   TestAccEC2InternetGateway_disappears
=== PAUSE TestAccEC2InternetGateway_disappears
=== RUN   TestAccEC2InternetGateway_Attachment
=== PAUSE TestAccEC2InternetGateway_Attachment
=== RUN   TestAccEC2InternetGateway_Tags
=== PAUSE TestAccEC2InternetGateway_Tags
=== CONT  TestAccEC2InternetGateway_basic
=== CONT  TestAccEC2InternetGateway_Tags
=== CONT  TestAccEC2InternetGateway_Attachment
=== CONT  TestAccEC2InternetGateway_disappears
--- PASS: TestAccEC2InternetGateway_disappears (12.10s)
--- PASS: TestAccEC2InternetGateway_basic (16.27s)
--- PASS: TestAccEC2InternetGateway_Tags (34.96s)
--- PASS: TestAccEC2InternetGateway_Attachment (45.39s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/ec2	48.975s

[zhelding]

LGTM! 🚀

[github-actions]

This functionality has been released in v3.66.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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