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/platform-browser-dynamic/src/compiler_factory.ts

@@ -160,7 +160,7 @@ export class JitCompilerFactory implements CompilerFactory {
       useJit: true,
       defaultEncapsulation: ViewEncapsulation.Emulated,
       missingTranslation: MissingTranslationStrategy.Warning,
-      enableLegacyTemplate: true,
+      enableLegacyTemplate: false,
     };
 
     this._defaultOptions = [compilerOptions, ...defaultOptions];