Fix PHP 8.1/8.2 deprecation warnings

[thelovekesh]

Summary

Update AMP_HTTP::send_header() to pass the third param as int in header() which was causing this deprecation warning:

PHP Deprecated:  header(): Passing null to parameter #3 ($response_code) of type int is deprecated in C:\Users\win 10\Local Sites\php8amp\app\public\wp-content\plugins\amp\includes\class-amp-http.php on line 95

Fixes #

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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[thelovekesh]

Closing in the favor of #7452