Include Java 19 in CI

Since Project Loom is a fairly significant internal architecture 
change within Java, it is probably worth running CI against it
as a baseline.

.github/workflows/ci.yml

@@ -29,7 +29,13 @@ jobs:
     # Definition of the build matrix
     strategy:
       matrix:
-        java: [8, 11, 17]
+        java:
+          # Run CI against LTS releases
+          - 8
+          - 11
+          - 17
+          # Run against JDK-19 to test project loom support works as expected.
+          - 19
         mock-maker: ['mock-maker-default', 'mock-maker-inline']
 
     # All build steps