https://wordpress.org/support/topic/http-equivcontent-type-attribute-in-meta-tag/

amp-workaround-meta-http-equiv-content-type.php

<?php
/**
 * AMP Workaround meta[http-equiv=Content-Type]
 *
 * @package   AMP_Workaround_Meta_Content_Type
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Workaround meta[http-equiv=Content-Type]
 * Description: Fix validation error caused by a theme using meta[http-equiv=Content-Type].
 * Plugin URI: https://gist.github.com/westonruter/d18fd37fb9a58a26ae375a0cb9570831
 * Version: 0.1.0
 * Author: Weston Ruter, Google
 * Author URI: https://weston.ruter.net/
 * License: GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * Gist Plugin URI: https://gist.github.com/westonruter/d18fd37fb9a58a26ae375a0cb9570831
 */

namespace AMP_Workaround_Meta_Content_Type;

add_filter(
	'amp_content_sanitizers',
	function ( $sanitizers ) {
		if ( ! class_exists( 'AMP_Workaround_Meta_Content_Type_Sanitizer' ) ) {
			class Meta_Content_Type_Sanitizer extends \AMP_Base_Sanitizer {
				function sanitize() {
					$xpath = new \DOMXPath( $this->dom );
					foreach ( $xpath->query( '/html/head/meta[ @http-equiv ]' ) as $meta ) {
						if ( strtolower( $meta->getAttribute( 'http-equiv' ) ) === 'content-type' ) {
							$meta->parentNode->removeChild( $meta );
						}
					}
				}
			}
		}
		$sanitizers[ __NAMESPACE__ . '\Meta_Content_Type_Sanitizer' ] = [];
		return $sanitizers;
	}
);

Installation instructions: https://gist.github.com/westonruter/6110fbc4bef0c4b8c021a112012f7e9c