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

aio/content/examples/template-syntax/src/app/app.component.html

@@ -78,11 +78,11 @@
 
 <p>Template input variable expression context (let hero)</p>
 <!-- template hides the following; plenty of examples later -->
-<template>
+<ng-template>
   <!-- #docregion context-var -->
   <div *ngFor="let hero of heroes">{{hero.name}}</div>
   <!-- #enddocregion context-var -->
-</template>
+</ng-template>
 
 <p>Template reference variable expression context (#heroInput)</p>
 <div (keyup)="0" class="context">