feat(compiler): set enableLegacyTemplate to false by default (#18756)

BREAKING CHANGE: the compiler option `enableLegacyTemplate` is now disabled by default as the `<template>` element has been deprecated since v4. Use `<ng-template>` instead. The option `enableLegacyTemplate` and the `<template>` element will both be removed in Angular v6.
PR Close #18756

packages/compiler/src/aot/compiler_factory.ts

@@ -65,7 +65,7 @@ export function createAotCompiler(compilerHost: AotCompilerHost, options: AotCom
   const config = new CompilerConfig({
     defaultEncapsulation: ViewEncapsulation.Emulated,
     useJit: false,
-    enableLegacyTemplate: options.enableLegacyTemplate !== false,
+    enableLegacyTemplate: options.enableLegacyTemplate === true,
     missingTranslation: options.missingTranslation,
     preserveWhitespaces: options.preserveWhitespaces,
   });