Plugin declares compat 1.2.20 but audit_syslog.php needs PHP 8.0

[somethingwithproof]

INFO declares compat = 1.2.20, and Cacti 1.2.x supports PHP 7.4. Two files on
develop use PHP 8.0 syntax, so installing on such a server fails at parse time
rather than reporting a version mismatch.

Linting the tree on a pinned php:7.4-cli container:

Parse error: syntax error, unexpected '|', expecting variable in ./phpstan/stubs/cacti.stubs.php on line 94
Parse error: syntax error, unexpected '|', expecting '{' in ./audit_syslog.php on line 623

audit_syslog.php:623:

function audit_syslog_fwrite(mixed $socket, string $message, string &$warning = ''): int|false {

mixed and int|false are both 8.0+. The stub file is dev-only and not loaded at
runtime, so audit_syslog.php is the one that matters.

These arrived with #63. tests/Security/Php74CompatibilityTest.php was removed in the
same change and replaced by tests/Security/Php81SyntaxTest.php, which states the
plugin targets 8.1+. The CI matrix runs 8.1 through 8.4, so nothing exercises the
floor that INFO advertises.

Two ways to close the gap, depending on which is intended:

• If 8.0+ is the intended floor, raise compat to a Cacti release whose PHP floor
  matches, note the requirement in README.md, and add a PHP_VERSION_ID guard in
  setup.php so an old server gets a message instead of a fatal.
• If 1.2.20 support is still wanted, restore 7.4-compatible signatures in
  audit_syslog.php and add a lint job on 7.4 to keep it that way.

Happy to send a PR for either once you pick.