Bump minimal Coq version to 8.9.0

This is required to have List.repeat in the standard library (next commit).

configure

@@ -503,14 +503,14 @@ missingtools=false
 echo "Testing Coq... " | tr -d '\n'
 coq_ver=$(${COQBIN}coqc -v 2>/dev/null | sed -n -e 's/The Coq Proof Assistant, version \([^ ]*\).*$/\1/p')
 case "$coq_ver" in
-  8.8.0|8.8.1|8.8.2|8.9.0|8.9.1|8.10.0|8.10.1|8.10.2|8.11.0|8.11.1|8.11.2|8.12.0|8.12.1|8.12.2|8.13.0|8.13.1)
+  8.9.0|8.9.1|8.10.0|8.10.1|8.10.2|8.11.0|8.11.1|8.11.2|8.12.0|8.12.1|8.12.2|8.13.0|8.13.1)
         echo "version $coq_ver -- good!";;
   ?*)
         echo "version $coq_ver -- UNSUPPORTED"
         if $ignore_coq_version; then
             echo "Warning: this version of Coq is unsupported, proceed at your own risks."
         else
-            echo "Error: CompCert requires a version of Coq between 8.8.0 and 8.13.1"
+            echo "Error: CompCert requires a version of Coq between 8.9.0 and 8.13.1"
             missingtools=true
         fi;;
   "")