config: update mediawiki_runtime_image to use PHP 8.1
Now that production uses the php8.1-fpm-multiversion-base image, switch scap's default mediawiki_runtime_image to match.
Bug: T390225
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Now that production uses the php8.1-fpm-multiversion-base image, switch scap's default mediawiki_runtime_image to match.
Bug: T390225