nixos/jenkins: Switch from JDK21 to JDK25

JDK 25, released in September 2025, is the recent LTS release and
received support from Jenkins 2.541. So use that by default from now on.

nixos/modules/services/continuous-integration/jenkins/default.nix

@@ -83,13 +83,13 @@ in
 
       package = lib.mkPackageOption pkgs "jenkins" { };
 
-      javaPackage = lib.mkPackageOption pkgs "jdk21" { };
+      javaPackage = lib.mkPackageOption pkgs "jdk25" { };
 
       packages = lib.mkOption {
         default = [
           pkgs.stdenv
           pkgs.git
-          pkgs.jdk21
+          pkgs.jdk25
           config.programs.ssh.package
           pkgs.nix
         ];