Renew or replace GPG key used for signing releases

[lrytz]

The key we currently use is expired (for a long time).

• Add PGP keys used by scala-lang to the security policy scala-lang#1672
• scala/scala@b52454d

I did not manage to recover the private key. If noone else has it, we'll have to create a new one. Or use the same that's used for signing Scala 3 releases?

[hamzaremmal]

@lrytz lets use The one used by Scala 3.