refactor(compiler): Drop support for the deprecated <template>. Use <ng-template> instead (#22783)

BREAKING CHANGE:

The `<template>` tag was deprecated in Angular v4 to avoid collisions (i.e. when
using Web Components).

This commit removes support for `<template>`. `<ng-template>` should be used
instead.

BEFORE:

    <!-- html template -->
    <template>some template content</template>

    # tsconfig.json
    {
      # ...
      "angularCompilerOptions": {
        # ...
        # This option is no more supported and will have no effect
        "enableLegacyTemplate": [true|false]
      }
    }

AFTER:

    <!-- html template -->
    <ng-template>some template content</ng-template>

PR Close #22783

modules/benchmarks/src/old/naive_infinite_scroll/app.ts

@@ -25,9 +25,9 @@ import {ScrollAreaComponent} from './scroll_area';
     <div style="display: flex">
       <scroll-area id="testArea"></scroll-area>
     </div>
-    <div template="ngIf scrollAreas.length > 0">
+    <div *ngIf="scrollAreas.length > 0">
       <p>Following tables are only here to add weight to the UI:</p>
-      <scroll-area template="ngFor let scrollArea of scrollAreas"></scroll-area>
+      <scroll-area *ngFor="let scrollArea of scrollAreas"></scroll-area>
     </div>
   </div>`
 })

modules/benchmarks/src/old/naive_infinite_scroll/cells.ts

@@ -62,7 +62,7 @@ export class Stage {
   directives: [NgFor],
   template: `
       <div [style.width.px]="cellWidth">
-          <button template="ngFor let stage of stages"
+          <button *ngFor="let stage of stages"
                   [disabled]="stage.isDisabled"
                   [style.background-color]="stage.backgroundColor"
                   on-click="setStage(stage)">

modules/benchmarks/src/old/naive_infinite_scroll/scroll_area.ts

@@ -25,7 +25,7 @@ import {ScrollItemComponent} from './scroll_item';
             <div id="padding"></div>
             <div id="inner">
                 <scroll-item
-                    template="ngFor let item of visibleItems"
+                    *ngFor="let item of visibleItems"
                     [offering]="item">
                 </scroll-item>
             </div>