Simplify copyright headers

[mattsb42-aws]

We no longer need to include year statements in our file header copyright statements. We also no longer need to include the full license statement, and can simply reference the SPDX identifier.

For example, rather than:

# Copyright 2017 Amazon.com, Inc. or its affiliates. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License"). You
# may not use this file except in compliance with the License. A copy of
# the License is located at
#
# http://aws.amazon.com/apache2.0/
#
# or in the "license" file accompanying this file. This file is
# distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF
# ANY KIND, either express or implied. See the License for the specific
# language governing permissions and limitations under the License.

we can simply include:

# Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
# SPDX-License-Identifier: Apache-2.0

We should update our repos for simplicity and consistency and add checks in our CI to verify that the header lines are present in every source file.