Ignore dependabot security alert

[sanket1729]

https://github.com/ElementsProject/rust-elements/security/dependabot/1 was dismissed by me with the note "Severe for this project". serde_cbor was only used in tests. serde_cbor was only used in test.

Opening an issue for people to double check

[apoelstra]

Good idea, thanks!