trusted proxies validation is invalid and allows nonsense values

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Steps to reproduce

1. add trusted_proxies with IPv6 syntax
2. create invalid notify_push config e.g. wrong trusted_proxies or missing headers
3. perform either occ notify_push:setup ... or occ notify_push:self-test
4. a warning about as invalid IPv6 proxy is shown

Expected behaviour

according to docs https://docs.nextcloud.com/server/stable/admin_manual/configuration_server/reverse_proxy_configuration.html reverse proxy could be IPv4 addresses and ranges in CIDR notation, IPv6 addresses and ranges in CIDR notation. the check should reflect this fact.

• IPv6 syntax should be valid
• IPv4 CIDR syntax should be verified properly (current regex allows nonsense values like 333.444.555.666/77)

Actual behaviour

🗴 push server is not a trusted proxy by Nextcloud or another proxy in the chain.
  Nextcloud resolved the following client address for the test request: "Invalid response when testing if the push server is a trusted proxy: invalid IP address syntax" instead of the expected "1.2.3.4" test value.
  The following trusted proxies are currently configured: "172.16.0.0/12", "192.168.0.0/16", "10.0.0.0/8", "fc00::/7", "fe80::/10", "2001:db8::/32"
    of which the following seem to be invalid: "fc00::/7", "fe80::/10", "2001:db8::/32"
  The following x-forwarded-for header was received by Nextcloud: ""
    from the following remote:

the problem results from $cidrre = '/^([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3})\/([0-9]{1,2})$/'; regex used in SelfTest.php which doesn't match IPv6 (and is not strict enough to match only valid IPv4 syntax)

https://github.com/nextcloud/notify_push/blob/f71fcbc1a0b348d7b4417bb65cb624bd79eb22bc/lib/SelfTest.php#L240C1-L248C3

private function isValidProxyConfig(string $proxyConfig): bool {
	$cidrre = '/^([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3})\/([0-9]{1,2})$/';

	if (filter_var($proxyConfig, FILTER_VALIDATE_IP) !== false) {
		return true;
	} else {
		return (bool)preg_match($cidrre, $proxyConfig);
	}
}

there are better regex like https://uibakery.io/regex-library/ip-address-regex-php or better check functions like https://gist.github.com/pavinjosdev/cb1d636ea9dc2bd201d54107d10650c5

The validation is in lib/SelfTest.php, in isValidProxyConfig; start there and reproduce the issue with occ notify_push:setup or occ notify_push:self-test using the trusted proxy examples. Done means valid IPv6 and IPv4 CIDR values are accepted while malformed IPv4 addresses and prefixes are rejected.