resource/aws_cognito_user_group: Fix role_arn updates

[bflad]

Fixes #4236

Previously (before code change):

make testacc TEST=./aws TESTARGS='-run=TestAccAWSCognitoUserGroup_RoleArn'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSCognitoUserGroup_RoleArn -timeout 120m
=== RUN   TestAccAWSCognitoUserGroup_RoleArn
--- FAIL: TestAccAWSCognitoUserGroup_RoleArn (18.61s)
	testing.go:518: Step 1 error: Error applying: 1 error(s) occurred:

		* aws_cognito_user_group.main: 1 error(s) occurred:

		* aws_cognito_user_group.main: Error updating Cognito User Group: InvalidParameter: 1 validation error(s) found.
		- minimum field size of 20, UpdateGroupInput.RoleArn.

FAIL
FAIL	github.com/terraform-providers/terraform-provider-aws/aws	18.650s
make: *** [testacc] Error 1

Now:

make testacc TEST=./aws TESTARGS='-run=TestAccAWSCognitoUserGroup_RoleArn'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSCognitoUserGroup_RoleArn -timeout 120m
=== RUN   TestAccAWSCognitoUserGroup_RoleArn
--- PASS: TestAccAWSCognitoUserGroup_RoleArn (22.91s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	22.946s

[bflad]

This has been released in version 1.15.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

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