fix some ecj warnings:

[jukzi]

set discouragedReference=ignore in snippets
set UTF-8 encoding
use try-with-resource
enable explicitlyClosedAutoCloseable=warning

[github-actions]

Test Results

     299 files  ±0       299 suites  ±0   5m 29s ⏱️ -11s
  4 095 tests ±0    4 087 ✔️ ±0    8 💤 ±0  0 ❌ ±0 
12 197 runs  ±0  12 124 ✔️ ±0  73 💤 ±0  0 ❌ ±0 

Results for commit a799cd7. ± Comparison against base commit 171e646.

♻️ This comment has been updated with latest results.

[akurtakov]

Could this be related to the failure https://ci.eclipse.org/platform/job/eclipse.platform.releng.aggregator/job/PR-1531/10/console

09:26:43  [INFO] Compiling 452 source files to /home/jenkins/agent/workspace/atform.releng.aggregator_PR-1531/eclipse.platform.swt.binaries/bundles/org.eclipse.swt.gtk.linux.aarch64/target/classes using Eclipse Compiler for Java(TM) 3.37.0.v20231130-2255
09:26:44  [WARNING] /home/jenkins/agent/workspace/atform.releng.aggregator_PR-1531/eclipse.platform.swt.binaries/bundles/org.eclipse.swt.gtk.linux.aarch64/src/org/eclipse/swt/widgets/Display.java:[5992] 
09:26:44  	T[] t = (T[]) new Object[1];
09:26:44  	        ^^^^^^^^^^^^^^^^^^^
09:26:44  Type safety: Unchecked cast from Object[] to T[]
09:26:44  [WARNING] /home/jenkins/agent/workspace/atform.releng.aggregator_PR-1531/eclipse.platform.swt.binaries/bundles/org.eclipse.swt.gtk.linux.aarch64/src/org/eclipse/swt/widgets/Display.java:[6003] 
09:26:44  	E e = (E) ex[0];
09:26:44  	      ^^^^^^^^^
09:26:44  Type safety: Unchecked cast from Object to E

[akurtakov]

No seems to be eclipse-jdt/eclipse.jdt.core#1661 (comment)