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

tools/public_api_guard/core/core.d.ts

@@ -112,7 +112,6 @@ export declare type CompilerOptions = {
     defaultEncapsulation?: ViewEncapsulation;
     providers?: StaticProvider[];
     missingTranslation?: MissingTranslationStrategy;
-    enableLegacyTemplate?: boolean;
     preserveWhitespaces?: boolean;
 };