radvd update to 2.21

[Neustradamus]

Dear @IPv6-mPvD team, @Oryon, @evyncke,

Can you update radvd to 2.21 with CVE fixes?

• https://github.com/radvd-project/radvd/releases
• https://github.com/radvd-project/radvd/tags

Improvements are welcome in upstream.

Thanks in advance.