PHP 7.3 won't be supported by upstream for the entire life cycle of
the 21.11 release.
Also drop the pcre' alias since it isn't needed anymore since we don't
need different pcre versions anymore.
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
