Update NEST package.py

Another thing I noticed while playing on Jupyter. We will discuss internally how we want to handle deprecation in Spack. Officially only the last two releases are supported (see policy).

packages/nest/package.py

@@ -23,7 +23,7 @@ class Nest(CMakePackage):
     version('3.0',    sha256='d481ea67f3251fe3aadf5252ab0a999172f0cd5536c5985366d271d772e686e6')
     patch('2021-07-17_fix-pyexecdir.patch', when='@3.0')
     version('2.20.1', sha256='df3d32b5899d5d444f708037b290f889ac6ff8eae6b7be9e9faee2c0d660d8e5')
-    version('2.20.0', sha256='40e33187c22d6e843d80095b221fa7fd5ebe4dbc0116765a91fc5c425dd0eca4')
+    version('2.20.0', sha256='40e33187c22d6e843d80095b221fa7fd5ebe4dbc0116765a91fc5c425dd0eca4', deprecated=True)
     version('2.18.0', sha256='7295c936fbdd5486395b06f54f0d4d35d9a1b6ee50b7b844186ec2c92de641d1')
     version('2.16.0', sha256='abfeb61719dec54da9477be035bef1d9d764f4e7663f63f6a6d9211f967e0490')
     version('2.14.0', sha256='d6316d6c9153100a3220488abfa738958c4b65bf2622bd15540e4aa81e79f17f')