Update .gitlab-ci.yml: use public runners with texlive:latest image

.gitlab-ci.yml

@@ -2,23 +2,19 @@ stages:
   - build
   - pages
 
-default:
-    tags:
-        - latex
-
 build-document:
   stage: build
+  image:
+      name: texlive/texlive:latest
   script:
       - .ci/build_document.sh
-  variables:
-      GIT_SUBMODULE_STRATEGY: recursive
   artifacts:
     paths:
         - public/
   only:
       - tags
       - branches
-        #      - merge_requests
+      - merge_requests
 
 
 pages:
@@ -30,5 +26,4 @@ pages:
         - echo "Deploying to GitLab pages"
         - test -f public/Matboj_Regelwerk.pdf
     only:
-        - master
         - main