Add a link to known security vulnerabilities

[MelissaAutumn]

As suggested here: https://bugzilla.mozilla.org/show_bug.cgi?id=1898616

We should probably have this link somewhere on our site. Possibly the footer, but if that is too crowded then the releases page will do.

[Sancus]

Should be on the releases page, I think. Having it as a front page link, even in the footer, seems very strangely prominent for something like that.

TBH I don't really see how you'd even get to the 3.1 page... I guess https://www.mozilla.org/en-US/security/known-vulnerabilities/ isn't super well designed since the old versions take up more space than the current ones, but they are pretty clearly labeled old versions.