Dropping PHP 7.4 and 8.0 support

Due to an upgrade from Symfony 5.4 to 6.4, Postmill now requires PHP 8.1 at minimum.

The last commit to support PHP 7.4 and 8.0 is a5e974b73b, made on 29 June 2024. If a major security issue is discovered, the fix might be backported to this version.

If you are using an old PHP version, your choices are:

• Upgrade.

  Ubuntu 22.04 and Debian 12 ship a PHP version that’s sufficiently recent. If upgrading these distros isn’t an option, you can get a recent PHP on an older distro using the deb.sury.org repository.

  For Red Hat and its derivatives, you can use Remi’s RPM repository to obtain a recent PHP.

• Do nothing.

  This is an option we unironically recommend. Presumably your existing installation works fine, why change it? Ancient versions of Postmill have been known to run without hiccup for years.