README.md: update actions used by example action file

Author: fingolfin

README.md

@@ -34,15 +34,15 @@ jobs:
     if: github.base_ref == github.event.repository.default_branch
     runs-on: ubuntu-latest
     steps:
-    - uses: julia-actions/setup-julia@v1
+    - uses: julia-actions/setup-julia@v2
       with:
         version: '1'
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: julia-actions/julia-buildpkg@v1
     - uses: julia-actions/julia-invalidations@v1
       id: invs_pr
 
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.repository.default_branch }}
     - uses: julia-actions/julia-buildpkg@v1