resource/aws_instance: Bump deletion timeout to 20mins

[radeksimko]

Similar to #3435

This is to address the following test failures:

=== RUN   TestAccAWSInstance_sourceDestCheck
--- FAIL: TestAccAWSInstance_sourceDestCheck (1416.49s)
    testing.go:573: Error destroying resource! WARNING: Dangling resources
        may exist. The full state and error is shown below.
        
        Error: Error applying: 1 error(s) occurred:
        
        * aws_instance.foo (destroy): 1 error(s) occurred:
        
        * aws_instance.foo: Error waiting for instance (i-0fa76ebc595aa2792) to terminate: timeout while waiting for state to become 'terminated' (last state: 'shutting-down', timeout: 10m0s)

=== RUN   TestAccAWSInstancesDataSource_tags
--- FAIL: TestAccAWSInstancesDataSource_tags (760.44s)
    testing.go:573: Error destroying resource! WARNING: Dangling resources
        may exist. The full state and error is shown below.
        
        Error: Error applying: 1 error(s) occurred:
        
        * aws_instance.test[3] (destroy): 1 error(s) occurred:
        
        * aws_instance.test.3: Error waiting for instance (i-0aac9db15df3d1648) to terminate: timeout while waiting for state to become 'terminated' (timeout: 10m0s)

[bflad]

LGTM

[ghost]

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