ci: Start testing with git v2.31.0

Git v2.31.0 has been released on March 15th, and Gitaly will probably
switch soonish to make that version the minimum required version. While
we've already been testing it for the past weeks via our nightly jobs,
this commit switches some of our CI jobs over to use git v2.31.0 instead
of v2.30.1. Note that this commit doesn't yet switch over our own
minimum required version of v2.29.0 -- this will happen at a later stage
when we've started to adjust build images across the different projects.

.gitlab-ci.yml

@@ -121,7 +121,7 @@ build:
       - GO_VERSION: [ "1.14", "1.15" ]
         GIT_VERSION: [ "v2.29.0" ]
       - GO_VERSION: "1.15"
-        GIT_VERSION: "v2.30.1"
+        GIT_VERSION: "v2.31.0"
 
 binaries:
   <<: *cache_definition
@@ -169,7 +169,7 @@ test:
         GIT_VERSION: "v2.29.0"
         TARGET: [ test-with-proxies, test-with-praefect, race-go ]
       - GO_VERSION: "1.15"
-        GIT_VERSION: "v2.30.1"
+        GIT_VERSION: "v2.31.0"
         TARGET: test
 
 nightly:git: