Update GHA

Author: jeroen

.github/workflows/R-CMD-check.yaml

@@ -22,7 +22,6 @@ jobs:
           - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
           - {os: ubuntu-22.04,   r: 'release'}
           - {os: ubuntu-20.04,   r: 'release'}
-          - {os: ubuntu-18.04,   r: 'release'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
@@ -31,9 +30,6 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
-      - if: matrix.config.os == 'ubuntu-18.04'
-        run: sudo add-apt-repository ppa:cran/opencv-3.4
-
       - uses: r-lib/actions/setup-r@v2
         with:
           r-version: ${{ matrix.config.r }}