Add a security.txt file to php.net (#816)

This file implements the standard defined in RFC 9116 for a
machine-parsable format to aid in security vulnerability disclosure.

Of note:

1. We must include an Expires field, which the RFC suggests should be
   less than a year in the future. I have set it for the assumed date
   for GA of PHP 8.4/9.0. I recommend we update the expires time each
   year on this date, since it's already a date of significance for us.

2. I have signed it with my php.net release manager key. Since we
   publish our release manager keys, I'm recommending that a release
   manager for a currently supported version of PHP (at the time) be the
   one to digitally sign this file after making changes.

For more details about security.txt, see:

- https://securitytxt.org
- https://www.rfc-editor.org/rfc/rfc9116

.well-known/security.txt

@@ -0,0 +1,31 @@
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA256
+
+Contact: https://github.com/php/php-src/security/advisories/new
+Contact: mailto:security@php.net
+Expires: 2024-11-28T11:59:59.999Z
+Preferred-Languages: en
+Canonical: https://www.php.net/.well-known/security.txt
+Policy: https://github.com/php/php-src/security/policy
+
+# Signed by Ben Ramsey <ramsey@php.net> on 2023-09-29.
+
+# For instructions on how to update this file, read
+# <https://github.com/php/php-src/blob/master/docs/security-policies.md#making-changes-to-securitytxt>
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