provider/aws: Updates `aws_cloudformation_stack` Update timeout (#7997)
provider/aws: Updates `aws_cloudformation_stack` Update timeout (#7997)

Fixes #7996

The Create func was using the timeout that we were passing to the
resource. Update func was not.

```
% make testacc TEST=./builtin/providers/aws
% TESTARGS='-run=TestAccAWSCloudFormation_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSCloudFormation_ -timeout 120m
=== RUN   TestAccAWSCloudFormation_basic
--- PASS: TestAccAWSCloudFormation_basic (120.61s)
=== RUN   TestAccAWSCloudFormation_defaultParams
--- PASS: TestAccAWSCloudFormation_defaultParams (121.40s)
=== RUN   TestAccAWSCloudFormation_allAttributes
--- PASS: TestAccAWSCloudFormation_allAttributes (263.29s)
=== RUN   TestAccAWSCloudFormation_withParams
--- PASS: TestAccAWSCloudFormation_withParams (205.52s)
=== RUN   TestAccAWSCloudFormation_withUrl_withParams
--- PASS: TestAccAWSCloudFormation_withUrl_withParams (402.71s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws
1113.552s
```
kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,nbaztec/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,Ninir/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,Ninir/terraform-provider-aws,charles-at-geospock/terraform-provider-aws