Drop support for Coq 8.20 and Rocq 9.0

Author: proux01

.github/workflows/coq-action.yml

@@ -19,10 +19,9 @@ jobs:
     strategy:
       matrix:
         image:
-          - mathcomp/mathcomp:2.1.0-coq-8.20
-          - mathcomp/mathcomp:2.1.0-coq-dev
-          - mathcomp/mathcomp-dev:coq-8.20
-          - mathcomp/mathcomp-dev:coq-dev
+          - mathcomp/mathcomp:2.4.0-rocq-prover-9.1
+          - mathcomp/mathcomp-dev:rocq-prover-9.1
+          - mathcomp/mathcomp-dev:rocq-prover-dev
       fail-fast: false
     steps:
       - uses: actions/checkout@v2