Revert "test Coq beta branch (since master is broken)"

This reverts commit 1226aa9c.
https://github.com/coq/coq/issues/13732 is supposed to be fixed; let's see.

.gitlab-ci.yml

@@ -36,7 +36,7 @@ build-coq.8.12.0:
 build-iris.dev:
   <<: *template
   variables:
-    OPAM_PINS: "coq version 8.13.dev   coq-stdpp.dev git git+https://gitlab.mpi-sws.org/iris/stdpp.git#$STDPP_REV  coq-iris.dev git git+https://gitlab.mpi-sws.org/iris/iris.git#$IRIS_REV   coq-iris-heap-lang.dev git git+https://gitlab.mpi-sws.org/iris/iris.git#$IRIS_REV"
+    OPAM_PINS: "coq version dev   coq-stdpp.dev git git+https://gitlab.mpi-sws.org/iris/stdpp.git#$STDPP_REV  coq-iris.dev git git+https://gitlab.mpi-sws.org/iris/iris.git#$IRIS_REV   coq-iris-heap-lang.dev git git+https://gitlab.mpi-sws.org/iris/iris.git#$IRIS_REV"
   except:
   only:
   - triggers