Change copyright statement and associated test

Migrated Jira Issue CDDSO-431
Jira creation date 30/03/2024 10:24
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We currently have a dated copyright statement at the top of every file. Looking at Cylc e.g. [https://github.com/cylc/cylc-flow/blob/master/cylc/flow/async_util.py|https://github.com/cylc/cylc-flow/blob/master/cylc/flow/async_util.py|smart-link] there is a header without date.

We should consider changing ours to

```# THIS FILE IS PART OF CDDS
# Copyright (C) British Crown (Met Office) & Contributors.
# Licenced under the BSD 3 clause license https://opensource.org/license/bsd-3-clause
# See the LICENSE file in the top level of this repository for full details.```

I don’t know whether we can get away with lines 3 & 4 – need to enquire as to whether we need full license details in every file.

Dates would then be updated in the LICENSE file only, and the unit tests would need to be amended
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Start by reviewing the LICENSE file and inventorying the dated copyright headers across the repository, then inspect the unit tests that assert their contents. Confirm the required per-file license wording before changing the headers; done means the headers and associated tests match the agreed statement and dates remain maintained only in LICENSE.

Written by the indexing model from the issue text.