[gtkmm-documentation] CI: Install meson == 0.56.0

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 5504d49a1b6784527881eb7488ad523873ce9c22
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Sat Feb 19 11:00:02 2022 +0100

    CI: Install meson == 0.56.0
    
    The latest version of gnome.yelp() does not work with a generated
    XML input file (index.docbook, generated from index-in.docbook).

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 5eb35b8..1b12482 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -8,7 +8,7 @@ stages:
   before_script:
     - export PATH="$HOME/.local/bin:$PATH"
     - dnf install -y ${FEDORA_DEPS}
-    - pip install --user "meson>=0.56.0"
+    - pip install --user "meson==0.56.0"
 
 docs-validation:
   extends: .build-default