[PDE-Build] Use 1.8 as default javac target/source-level in scripts

ECJ does not support Java versions below 1.8 anymore.

Contributes to fixing
eclipse-platform/eclipse.platform.releng.aggregator#2197

build/org.eclipse.pde.build/src/org/eclipse/pde/internal/build/builder/ModelBuildScriptGenerator.java

@@ -1068,9 +1068,9 @@ private void generateCompilerSettings() throws CoreException {
 		script.printProperty(PROPERTY_PREREQ_COMPILE_LOG, Utils.getPropertyFormat(PROPERTY_BUILD_DIRECTORY) + "/prereqErrors.log"); //$NON-NLS-1$
 
 		if (javacSource == null)
-			script.printProperty(IXMLConstants.PROPERTY_JAVAC_SOURCE, "1.3"); //$NON-NLS-1$
+			script.printProperty(IXMLConstants.PROPERTY_JAVAC_SOURCE, "1.8"); //$NON-NLS-1$
 		if (javacTarget == null)
-			script.printProperty(IXMLConstants.PROPERTY_JAVAC_TARGET, "1.2"); //$NON-NLS-1$  
+			script.printProperty(IXMLConstants.PROPERTY_JAVAC_TARGET, "1.8"); //$NON-NLS-1$  
 		if (bootClasspath == null) {
 			script.println("<condition property=\"dir_bootclasspath\" value=\"${java.home}/../Classes\">");//$NON-NLS-1$  
 			script.println("\t<and>"); //$NON-NLS-1$