Extending support for the NetBSD-7 branch

Typically, some time after releasing a new NetBSD major version (such as
NetBSD 9.0), we will announce the end-of-life of the N-2 branch, in this
case NetBSD-7.

We've decided to hold off on doing that to ensure our users don't feel
rushed to perform a major version update on any remote machines,
possibly needing to reach the machine if anything goes wrong.

Security fixes will still be made to the NetBSD-7 branch.

We hope you're all safe. Stay home.