Revert most of "test against Coq 8.8.0, update CI"

This reverts commit 45162e5a.
We have to wait until ssreflect makes an 8.8-compatible release.

.gitlab-ci.yml

@@ -32,11 +32,6 @@ build-coq.dev:
     OPAM_PINS: "coq version dev   coq-mathcomp-ssreflect version dev"
     VALIDATE: "1"
 
-build-coq.8.8.0:
-  <<: *template
-  variables:
-    OPAM_PINS: "coq version 8.8.0   coq-mathcomp-ssreflect version dev"
-
 build-coq.8.7.2:
   <<: *template
   variables:

README.md

@@ -6,7 +6,7 @@ This is the Coq development of the [Iris Project](http://iris-project.org).
 
 This version is known to compile with:
 
- - Coq 8.6.1 / 8.7.0 / 8.7.1 / 8.7.2 / 8.8.0
+ - Coq 8.6.1 / 8.7.0 / 8.7.1 / 8.7.2
  - Ssreflect 1.6.4
  - A development version of [std++](https://gitlab.mpi-sws.org/robbertkrebbers/coq-stdpp)