provider/aws: aws_redshift_cluster now allowspublicly_accessible to be modified

[stack72]

Fixes #5540

Also updated the AWS Go SDK from 1.1.9 -> 1.1.12 as this was required to
allow the new behavior for the Redshift API

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSRedshiftCluster' 2>~/tf.log
==> Checking that code complies with gofmt requirements...
/Users/stacko/Code/go/bin/stringer
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSRedshiftCluster -timeout 120m
=== RUN   TestAccAWSRedshiftCluster_basic
--- PASS: TestAccAWSRedshiftCluster_basic (651.26s)
=== RUN   TestAccAWSRedshiftCluster_publiclyAccessible
--- PASS: TestAccAWSRedshiftCluster_publiclyAccessible (752.17s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    1403.431s

[phinze]

LGTM

[ghost]

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