docs: update copyright years

Author: MightyCreak

README.md

@@ -60,6 +60,6 @@ is licensed under the [FSF-AP](https://www.gnu.org/prep/maintain/html_node/Licen
 license.
 
 Copyright (C) 2006-2019 Derrick Moser <[email protected]>  
-Copyright (C) 2015-2021 Romain Failliot <[email protected]>
+Copyright (C) 2015-2023 Romain Failliot <[email protected]>
 
 Icon made by [@jimmac](https://github.com/jimmac).

src/diffuse/constants.py

@@ -22,7 +22,7 @@
 
 APP_NAME: Final[str] = 'Diffuse'
 COPYRIGHT: Final[str] = '''{copyright} © 2006-2019 Derrick Moser
-{copyright} © 2015-2021 Romain Failliot'''.format(copyright=_("Copyright"))
+{copyright} © 2015-2023 Romain Failliot'''.format(copyright=_("Copyright"))
 WEBSITE: Final[str] = 'https://mightycreak.github.io/diffuse/'
 
 # Constants are set in main()