.well-known/security.txt: update Bug-Bounty field to comply with IANA

There now is an official [Bug-Bounty field][1] that has slightly
different semantics and syntax than what we suggested. So let's update
our field to comply with the official standard.

[1]: https://www.iana.org/assignments/security-txt-fields/security-txt-fields.xhtml

Author: maribu

.well-known/security.txt

@@ -3,4 +3,4 @@ Expires: 2028-01-13T10:09:55.000Z
 Policy: https://guide.riot-os.org/general/security/
 Preferred-Languages: en
 Encryption: https://riot-os.org/assets/keys/security.asc
-Bug-Bounty: None
+Bug-Bounty: False