flake.nix: use JDK 23 to run devshell Gradle

Since our current config (flake.lock) is using Gradle 8.12.1,
we can (and should) use the `java` override property to install jdk23
rather than the `javaToolchains` property.

(This will change when we move to Gradle 8.14 and JDK 24, but for now
this is the simplest and fastest to install configuration)

We are also moving from temurin-bin-23 which was a workaround
for a problem that seems to have been fixed.

These changes reduce the number of JDKs that need to be installed
from 3 down to 1. (Nixpkgs `gradle` currently defaults to jdk21.)

Author: msgilligan

flake.nix

@@ -31,11 +31,11 @@
         devShells.default = pkgs.mkShell {
           inputsFrom = with pkgs ; [ secp256k1 ];
           packages = with pkgs ; [
-                jdk23                # JDK 23 will be in $JAVA_HOME (and in javaToolchains)
+                jdk23                      # JDK 23 will be in PATH
                 # current jextract in nixpkgs is broken, see: https://github.com/NixOS/nixpkgs/issues/354591
-                # jextract             # jextract (Nix package) contains a jlinked executable and bundles its own JDK
-                (gradle.override {   # Gradle 8.x (Nix package) depends-on and directly uses JDK XX to launch Gradle itself
-                    javaToolchains = [ temurin-bin-23 ]; # Put JDK 23 in Gradle's javaToolchain configuration
+                # jextract                 # jextract (Nix package) contains a jlinked executable and bundles its own JDK
+                (gradle.override {         # Gradle 8.x (Nix package) runs using an internally-linked JDK
+                    java = jdk23;          # Run Gradle with this JDK
                 })
             ];
         };