doc: Replace apoelstra's GPG key by jonasnick's GPG key

I have verified the new key via other secure channels.

Author: real-or-random

SECURITY.md

@@ -9,7 +9,7 @@ The following keys may be used to communicate sensitive information to developer
 | Name | Fingerprint |
 |------|-------------|
 | Pieter Wuille | 133E AC17 9436 F14A 5CF1  B794 860F EB80 4E66 9320 |
-| Andrew Poelstra | 699A 63EF C17A D3A9 A34C  FFC0 7AD0 A91C 40BD 0091 |
+| Jonas Nick | 36C7 1A37 C9D9 88BD E825  08D9 B1A7 0E4F 8DCD 0366 |
 | Tim Ruffing | 09E0 3F87 1092 E40E 106E  902B 33BC 86AB 80FF 5516 |
 
 You can import a key by running the following command with that individual’s fingerprint: `gpg --recv-keys "<fingerprint>"` Ensure that you put quotes around fingerprints containing spaces.