Revert "abseil-cpp: 20250127.1 -> 20250512.1"

This reverts commit d3a1f2f9de.

abseil-cpp_202501 shouldn't become abseil-cpp_202505 silently, a
separate abseil-cpp_202505 attrset should be included.

Downstream abseil consumers are generally very picky about the version.
Maybe we should also do away with the abseil-cpp alias and be explicit
about the version everywhere, but that's unrelated to this revert.

pkgs/by-name/ab/abseil-cpp_202501/package.nix

@@ -10,13 +10,13 @@
 
 stdenv.mkDerivation (finalAttrs: {
   pname = "abseil-cpp";
-  version = "20250512.1";
+  version = "20250127.1";
 
   src = fetchFromGitHub {
     owner = "abseil";
     repo = "abseil-cpp";
     tag = finalAttrs.version;
-    hash = "sha256-eB7OqTO9Vwts9nYQ/Mdq0Ds4T1KgmmpYdzU09VPWOhk=";
+    hash = "sha256-QTywqQCkyGFpdbtDBvUwz9bGXxbJs/qoFKF6zYAZUmQ=";
   };
 
   cmakeFlags = [