HV-1969 Stop testing against JDK 20

It EOL'd on Sept 19th, 2023.
See https://endoflife.date/oracle-jdk

Also, we already test against the next version (21).

Author: yrodiere

Jenkinsfile

@@ -124,9 +124,6 @@ stage('Configure') {
 					// We want to enable preview features when testing newer builds of OpenJDK:
 					// even if we don't use these features, just enabling them can cause side effects
 					// and it's useful to test that.
-					new JdkBuildEnvironment(testJavaVersion: '20', testCompilerTool: 'OpenJDK 20 Latest',
-							testLauncherArgs: '--enable-preview',
-							condition: TestCondition.AFTER_MERGE),
 					new JdkBuildEnvironment(testJavaVersion: '21', testCompilerTool: 'OpenJDK 21 Latest',
 							testLauncherArgs: '--enable-preview',
 							condition: TestCondition.AFTER_MERGE),