[php-src] Issue #23990: Consider deferring the "is"/"let" identifier deprecation to PHP 8.7

Issue: https://github.com/php/php-src/issues/23990 Author: GrahamCampbell The deprecation of is and let as class names from the PHP 8.6 deprecations RFC was implemented in #23709. That PR was merged on 16 September, after feature freeze and during the RC phase. guzzle/promises has a public class GuzzleHttp\Promise\Is, which is used heavily in Guzzle and across the wider ecosystem. Loading it on PHP 8.6 now emits a deprecation notice (guzzle/promises#244). Renaming a public class is a breaking change for us, so we can only do it in a new major version. Because the deprecation landed this late, maintainers of affected libraries have had almost no time to prepare before 8.6.0 ships, and many users run with deprecations surfaced in tests or logs. I'd like to ask the release managers to consider moving this deprecation to PHP 8.7. That would give downstream projects a normal release cycle to plan the rename.