Doc: Update License Notice (#411)

- We do not need to cover year ranges (just the starting range).
- Update to exact wording of current notice texts.

LICENSE.txt

@@ -1,4 +1,4 @@
-ImpactX Copyright (c) 2022-2023, The Regents of the University of California,
+ImpactX Copyright (c) 2022, The Regents of the University of California,
 through Lawrence Berkeley National Laboratory (subject to receipt of
 any required approvals from the U.S. Dept. of Energy). All rights reserved.
 

README.md

@@ -326,11 +326,11 @@ ImpactX is supported by the CAMPA collaboration, a project of the U.S. Departmen
 
 ## Copyright Notice
 
-Copyright (c) 2022-2023, The Regents of the University of California, through Lawrence Berkeley National Laboratory (subject to receipt of any required approvals from the U.S. Dept. of Energy).
+ImpactX Copyright (c) 2022, The Regents of the University of California, through Lawrence Berkeley National Laboratory (subject to receipt of any required approvals from the U.S. Dept. of Energy).
 All rights reserved.
 
 If you have questions about your rights to use or distribute this software, please contact Berkeley Lab's Intellectual Property Office at [email protected].
 
-NOTICE. This Software was developed under funding from the U.S. Department of Energy and the U.S. Government consequently retains certain rights.  As such, the U.S. Government has been granted for itself and others acting on its behalf a paid-up, nonexclusive, irrevocable, worldwide license in the Software to reproduce, distribute copies to the public, prepare derivative works, and perform publicly and display publicly, and to permit others to do so.
+NOTICE. This Software was developed under funding from the U.S. Department of Energy and the U.S. Government consequently retains certain rights. As such, the U.S. Government has been granted for itself and others acting on its behalf a paid-up, nonexclusive, irrevocable, worldwide license in the Software to reproduce, distribute copies to the public, prepare derivative works, and perform publicly and display publicly, and to permit others to do so.
 
-Please see the full license agreement in [LICENSE.txt](LICENSE.txt), which is the `BSD-3-Clause-LBNL` license.
+Please see the full license agreement in [LICENSE.txt](LICENSE.txt). The SPDX license identifier is `BSD-3-Clause-LBNL`.