[11.x] Replace dead link in Security Policy

[Jubeki]

Seems like a dead link is used in the PGP Key Comment in the Security Policy: https://sela.io/pgp/

So I replaced it with a meaningful comment instead. Please check internally if the PGP Key is still valid for usage.

It would also be possible to simply delete the Comment itself.

[driesvints]

Thanks! Ugh, we'll need to update all SECURITY.md files now in every repo 😅

[Jubeki]

I can send in a PR to every Repo if you want. 😆

[driesvints]

@Jubeki don't want to put you through that but can't stop you if you want that 😅 would be very appreciated 👍

Let's wait first until Taylor merges this one though.

[Jubeki]

I have no problem with it (doesn't take that long) and I will do that after this is merged. 👍