feat: mention license in headers

[kevgo]

After some feedback, I have decided to mention the actual license in the file copyright headers. Apache recommends a lengthy header:

Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
software distributed under the License 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.    

I think this lengthy header is a relict from the 1970s when the open-source started to form and people had to over-communicate the idea. These days, everybody knows what the Apache license is, so I think a shorter form using the official SPDX-identifier is better since it is human-readable and machine-understandable.

// Copyright © 2021 Ory Corp
// SPDX-License-Identifier: Apache-2.0

Related Issue or Design Document

https://github.com/ory-corp/general/issues/602

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