Update irssi

Changes:

- https://github.com/jessfraz/irssi/commit/1581d0c: Update to Alpine 3.21
- https://github.com/jessfraz/irssi/commit/11dd5ac: Update README
- https://github.com/jessfraz/irssi/commit/47d6f81: Update `generate-stackbrew-library.sh` to support `BASHBREW_LIBRARY` for easier cascading updates
- https://github.com/jessfraz/irssi/commit/9a596c8: Update to actions/checkout@v4 🙃

library/irssi

@@ -1,4 +1,4 @@
-# this file is generated via https://github.com/jessfraz/irssi/blob/0643d4116d06af815d92aba78eaf1278ce49364a/generate-stackbrew-library.sh
+# this file is generated via https://github.com/jessfraz/irssi/blob/47d6f81f1b7da4d6e4b9eb0529085f51421562fb/generate-stackbrew-library.sh
 
 Maintainers: Jessie Frazelle <[email protected]> (@jessfraz),
              Tianon Gravi <[email protected]> (@tianon)
@@ -9,7 +9,7 @@ Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, mips64le, ppc64le, s390x
 GitCommit: b8ea417aaa1a29a6003756627d748450a5bf6abe
 Directory: debian
 
-Tags: 1.4.5-alpine, 1.4-alpine, 1-alpine, alpine, 1.4.5-alpine3.20, 1.4-alpine3.20, 1-alpine3.20, alpine3.20
+Tags: 1.4.5-alpine, 1.4-alpine, 1-alpine, alpine, 1.4.5-alpine3.21, 1.4-alpine3.21, 1-alpine3.21, alpine3.21
 Architectures: amd64, arm32v6, arm32v7, arm64v8, i386, ppc64le, riscv64, s390x
-GitCommit: ce043e84f00a74532313e8a6203bc8b96194c28a
+GitCommit: 1581d0c09544a0bd8de42ba3d185956be6a9935c
 Directory: alpine