Add security.txt

- Add scripts/security-txt.sh
  to generate a security.txt file
- Add the current security.txt
- Extend verify.sh to verify security.txt by default
  and cross-check that it expires together with the signing key
- Describe security.txt in OPENPGP.md and README.md

security.txt

@@ -0,0 +1,18 @@
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA512
+
+Canonical: https://www.alexander-hess.com/.well-known/security.txt
+Canonical: https://www.webartifex.de/.well-known/security.txt
+Contact: mailto:ah@alexander-hess.com
+Contact: mailto:alexander@webartifex.de
+Encryption: https://www.alexander-hess.com/public-key.asc
+Encryption: https://www.webartifex.de/public-key.asc
+Expires: 2027-01-31T11:00:29Z
+Preferred-Languages: en, de
+-----BEGIN PGP SIGNATURE-----
+
+iHUEARYKAB0WIQTW0Zid8fayP9d2KT20bttN/CaAXQUCaqcTnAAKCRC0bttN/CaA
+XXX/AP0c4y7eMZmGQc5GwCWQDAgTELVBZc6zBNyOjoThEH0qdgEAmahp+3716stJ
+iIosJWusNiz1ctVkEdS1WpzH3OfyEwQ=
+=joRB
+-----END PGP SIGNATURE-----