Let GDToolkit support AVIF image format

This issue is currently postponed on:

1. Bots having PHP instances that can fully support AVIF files - currently the bots' PHP, 8.1 and 8.2, cannot save files via imageavif(), the (bundled?) GD library is missing an appropriate codec supporting the write. (Also see https://bugs.php.net/bug.php?id=81217 , https://github.com/php/php-src/pull/7526 )

Problem/Motivation

AVIF image file format is getting traction in browsers' support.

This issue is proposing to implement support to this image format in Drupal core's GD Toolkit.

Done:

• libgd should support the AVIF format - DONE, first release supporting is 2.3.2, released on: March 6, 2021
• a PHP version supporting an IMAGETYPE_AVIF constant - upstream PR https://github.com/php/php-src/pull/5127 - supported with PHP 8.1
• a DrupalCI testbot container should be available for testing.
• #3239935: Refactor ToolkitGdTest
• a bot with a PHP 8.1. instance compiled with libavif support
• a PHP 8.2 bot to test getimagesize() on AVIF
• #3116611: Add a requirements check for GD support of allowed image types

Proposed resolution

Extend the GD toolkit to support AVIF too.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet