Set release date

Author: edolstra

doc/manual/source/SUMMARY.md.in

@@ -129,7 +129,7 @@
 - [Determinate Nix Release Notes](release-notes-determinate/index.md)
   - [Changes between Nix and Determinate Nix](release-notes-determinate/changes.md)
   - [Release 3.0.0 (2025-03-04)](release-notes-determinate/rl-3.0.0.md)
-  - [Release 3.1.0 (2025-??-??)](release-notes-determinate/rl-3.1.0.md)
+  - [Release 3.1.0 (2025-03-27)](release-notes-determinate/rl-3.1.0.md)
 - [Nix Release Notes](release-notes/index.md)
 {{#include ./SUMMARY-rl-next.md}}
   - [Release 2.27 (2025-03-03)](release-notes/rl-2.27.md)

doc/manual/source/release-notes-determinate/rl-3.1.0.md

@@ -1,3 +1,3 @@
-# Release 3.1.0 (2025-??-??)
+# Release 3.1.0 (2025-03-27)
 
 * Based on [upstream Nix 2.27.1](../release-notes/rl-2.27.md).