Switch minimum required version to JDK11

This mirrors the change in the main OpenRS2 repository. I think the
natives actually work all the way back to Java 6 (or maybe even
earlier). However, we're only routinely testing the build against JDK11
(plus legacy Java 6 on macOS).

README.md

@@ -12,7 +12,7 @@ jagmisc native libraries.
 Building OpenRS2 Natives requires the following pieces of software on all
 platforms:
 
-* [Java Development Kit][jdk] (version 8 or later)
+* [Java Development Kit][jdk] (version 11 or later)
 * [Apache Maven][maven] (version 3.3.9 or later)
 
 ### Platform-specific dependencies

shell.nix

@@ -6,7 +6,7 @@
     targetPkgs = pkgs: (
       with pkgs; [
         gcc
-        jdk8
+        jdk11
         libGL.dev
         maven
         xorg.libX11.dev