Revert "BLD: Set numpy latest supported version to 2.1"

This reverts commit 57cc1b6.

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist =
     py{38,39,310,311,312}-test{,-all}{,-dev,-latest,-oldest}{,-cov}
-    py{38,39,310,311,312}-test-numpy{116,117,118,119,120,121,122,123,124,125,126,20,21}
+    py{38,39,310,311,312}-test-numpy{116,117,118,119,120,121,122,123,124,125,126,20}
     py{38,39,310,311,312}-test-scipy{16,17,18,19,110,111,112,113,114}
     py{38,39,310,311,312}-test-astropy{40,41,42,43,50,51,52,53,60,61}
     build_docs
@@ -48,7 +48,6 @@ description =
     numpy125: with numpy 1.25.*
     numpy125: with numpy 1.26.*
     numpy20: with numpy 2.0.*
-    numpy21: with numpy 2.1.*
     scipy16: with scipy 1.6.*
     scipy17: with scipy 1.7.*
     scipy18: with scipy 1.8.*
@@ -84,7 +83,6 @@ deps =
     numpy125: numpy==1.25.*
     numpy125: numpy==1.26.*
     numpy20: numpy==2.0.*
-    numpy21: numpy==2.1.*
 
     scipy16: scipy==1.6.*
     scipy17: scipy==1.7.*
@@ -112,7 +110,7 @@ deps =
     dev: git+https://github.com/astropy/astropy.git#egg=astropy
 
     latest: astropy==6.1.*
-    latest: numpy==2.1.*
+    latest: numpy==2.0.*
     latest: scipy==1.14.*
 
     oldest: astropy==4.0.*