gitlab-ci: remove 'allow_failure: true' (MR 2327)

Now that it is all passing, let's keep it that way!

.gitlab-ci.yml

@@ -44,7 +44,6 @@ pytest:
 
 ruff:
   stage: lint
-  allow_failure: true
   script:
     - ".ci/ruff.sh"
 
@@ -55,7 +54,6 @@ shellcheck:
 
 vermin:
   stage: lint
-  allow_failure: true
   script:
     - ".ci/vermin.sh"
 
@@ -72,7 +70,6 @@ mr-settings:
 
 mypy:
   stage: lint
-  allow_failure: true
   script:
     - ".ci/mypy.sh"