ci(dependabot): don't get prompted for major version updates for github actions (#39733)

Major version updates require more preparation and testing, so we only
want to update these on our schedule / if actually deprecated, not be
prompted as soon as a new major version comes out. We mostly want to
keep patch/minor updates so we can be aware of any vulnerability fixes.

.github/dependabot.yml

@@ -4,3 +4,7 @@ updates:
     directory: '/'
     schedule:
       interval: 'weekly'
+    ignore:
+      - dependency-name: '*'
+        update-types:
+          - 'version-update:semver-major'