Clarify copyright notice

Since Logisim-evolution does not use a CLA to transfer external contributors' copyright to the Logisim-evolution organization, the copyright holder belongs to their respective author.

I believe using "Logisim-evolution developers" implies that it's copyrighted solely by the main developers of Logisim-evolution, or in other words, by the people working for Logisim-evolution. I suggest using one of the notices below:

• © 2001 Logisim-evolution contributors.
• © 2001 Logisim-evolution authors.

This clarifies that the code is copyrighted under the Logisim-evolution organization AND its respective contributors. Also, stating the date in the copyright implies "and later", and therefore the "2021" in the original copyright notice can be safely removed and as a benefit for it to be easier to maintain.

The Linux Foundation also suggests a similar format: https://www.linuxfoundation.org/blog/copyright-notices-in-open-source-software-projects/#:~:text=Instead%2C%20many%20LF,the%20named%20project.

Search the repository for the existing “Logisim-evolution developers” copyright notice and identify every occurrence. Review the proposed “contributors” or “authors” wording and the suggested removal of “2021”; done means the selected notice is updated consistently wherever it appears.