Actually update ZNC to 1.8.1

I forgot one important detail in #8141

library/znc

@@ -3,8 +3,8 @@ GitRepo: https://github.com/znc/znc-docker.git
 GitCommit: cb2cabf8d63bc992f530b0fb86d0ad9cf955819d
 Architectures: amd64, arm32v6, arm64v8
 
-Tags: 1.8.0, 1.8, latest
+Tags: 1.8.1, 1.8, latest
 Directory: full
 
-Tags: 1.8.0-slim, 1.8-slim, slim
+Tags: 1.8.1-slim, 1.8-slim, slim
 Directory: slim