ci: Temporarily disable CentOS 7 tests

Suspend the CI runs on CentOS 7, which fail due to a cryptic git error
connected with checking out the libshared submodule version. Might be
just a bug that is fixed in more recent git versions, which would
explain why other CI runs work fine.

.github/workflows/tests.yaml

@@ -6,9 +6,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - name: "Centos 7"
-            runner: ubuntu-latest
-            dockerfile: centos7
           - name: "Centos 8"
             runner: ubuntu-latest
             dockerfile: centos8