Remove Vue updates from Dependabot

Per the README, we want to keep using the same Vue version as MediaWiki
core, so let’s disable Dependabot updates for these packages.

.github/dependabot.yml

@@ -10,3 +10,9 @@ updates:
       schedule:
           interval: monthly
       open-pull-requests-limit: 15
+      ignore:
+          # use the same version of Vue and Vuex as in MediaWiki core
+          - dependency-name: "vue"
+          - dependency-name: "@vue/compiler-dom"
+          - dependency-name: "@vue/server-renderer"
+          - dependency-name: "vuex"