php80Extensions.apcu_bc: mark as broken

ZHF #122042
Failing Hydra build: https://hydra.nixos.org/build/142591498

`apcu_bc`[1] is a PHP extension which provides support for the older APC
caching-interface (i.e. `apc_fetch`, `apc_store` etc.) on newer PHP
versions. As the codebase wasn't touched since 2019 it's not really
surprising that it doesn't compile for PHP 8. Also, if people have
already managed to get their codebases running on PHP8, they're most
likely also using the newer APCu[2] interface.

I think it's okay to keep this as long as we support PHP 7.4 and drop
the package after that.

[1] https://pecl.php.net/package/apcu_bc
[2] https://www.php.net/manual/en/book.apcu.php

pkgs/development/php-packages/apcu_bc/default.nix

@@ -15,4 +15,5 @@ buildPecl {
   '';
 
   meta.maintainers = lib.teams.php.members;
+  meta.broken = lib.versionAtLeast php.version "8";
 }