* Remove obsolete `load` event handler in `wp-embed` since IE10+ support `DOMContentLoaded`.
* Replace obsolete use of `document.createElement('a')` in favor of the newer `URL` class (supported in all browsers but obsolete IE11).
* Remove obsolete IE10/IE11 code.
* Combine conditionals.
* Use `substring()` instead of deprecated `substr()` method.
* Eliminate the stipulation that `wp-embed.js` not include ampersands, considering this was put in place for WP<4.3 which now accounts for only 1.43% of sites. This includes the elimination of the `verify:wp-embed` grunt task.
Props westonruter, swissspidy.
Fixes#58974.
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