use ubuntu-latest for tvmbot runner

Author: mshr-h

.github/workflows/tvmbot.yml

@@ -21,7 +21,7 @@ jobs:
       pull-requests: write
       statuses: write
     if: ${{ github.event.issue.pull_request && github.repository == 'apache/tvm' }}
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - name: Run tvm-bot