[atkmm] CI: Select latest released version of libsigc++3

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 9b7c7b0f54f01ca35dad1bb654759e37f14f2dd7
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Sun Feb 27 09:14:53 2022 +0100

    CI: Select latest released version of libsigc++3

 .gitlab-ci.yml | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 3fd41f5..6924876 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,3 +1,5 @@
+# 2022-02-27: ubuntu:latest = 20.04, ubuntu:rolling = 21.10, ubuntu:devel = 22.04
+# See https://hub.docker.com/_/ubuntu
 image: ubuntu:devel
 
 stages:
@@ -50,7 +52,7 @@ build_deps:
     # Build and cache dependencies that can't be installed with apt.
     # Build libsigc++3
     - if test ! -d libsigc; then
-    - git clone --branch 3.0.7 --depth 1 https://github.com/libsigcplusplus/libsigcplusplus.git libsigc
+    - git clone --branch 3.2.0 --depth 1 https://github.com/libsigcplusplus/libsigcplusplus.git libsigc
     - cd libsigc
     - mkdir _build && cd _build
     - meson --prefix=/usr --libdir=lib -Dbuild-examples=false -Dbuildtype=release