[MAINT] Do not use copyright year ranges

[ndgrigorian]

This PR undoes use of copyright year ranges, using only the file creation year

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/2319/index.html

[coveralls]

coverage: 75.39%. remained the same — use-2020-as-copyright-year into master

[vlad-perevezentsev]

If we are switching to Copyright [yyyy] [name of copyright owner] format following the Apache License example could you also update sycl.pxd, _numpy_warnings.py and the remaining files in examples/c/ so that all copyright headers use the same style?

[ndgrigorian]

@vlad-perevezentsev
applied changes to those files

[vlad-perevezentsev]

@ndgrigorian Overall, LGTM.
But I am a bit concerned that for some files the copyright year does not match the file creation year probably because the copyright header was copied from existing files in the past.

I think the year should correspond to the year when the file was created.
For example, if _sycl_timer.py was introduced in 2021 then it should use Copyright 2021 Intel Corporation

Since checking this manually is quite time-consuming, I asked claude to analyze the file history.
Here are the results grouped by year:

copyright_years.md

[ndgrigorian]

copyright_years.md

this change is fine with me, my only thought was that we could move to a universal copyright date, but I think it would make sense to just use file creation year as well. I'll update the PR.