build(java-client): retarget JDK 21 -> 17 for Ignition 8.3 (v0.2.0)

Ignition 8.3 runs on JDK 17; the 0.1.2 client was compiled --release 21
(class-file v65) and would throw UnsupportedClassVersionError when loaded
in an Ignition module. Drop the toolchain and bytecode target to 17 so the
artifact loads on the gateway (a 17 build still runs on 21+). No source uses
Java 18-21 APIs, so this is a pure target change. Bump 0.1.2 -> 0.2.0.

Update the toolchain-pinning smoke test to assert feature()==17.
All 97 tests pass; all jar classes verified class-file v61. Roadmap G0.

clients/java/build.gradle

@@ -13,18 +13,20 @@ ext {
 
 subprojects {
     group = 'com.zb.mom.ww.mxgateway'
-    version = '0.1.2'
+    version = '0.2.0'
 
     pluginManager.withPlugin('java') {
         java {
+            // Retargeted 21 -> 17 so the artifact loads on Ignition 8.3's JDK 17
+            // (a 17-targeted build still runs on 21+). See roadmap G0.
             toolchain {
-                languageVersion = JavaLanguageVersion.of(21)
+                languageVersion = JavaLanguageVersion.of(17)
             }
         }
 
         tasks.withType(JavaCompile).configureEach {
             options.encoding = 'UTF-8'
-            options.release = 21
+            options.release = 17
         }
 
         tasks.withType(Test).configureEach {