GitHub CI: Use correct 8.10 version

The full tests that are run on the `1.6` branch accidentally refer to a
non-existing docker image for 8.10: PR #2201 introduces 8.10.7 as the
version for the docker image, but omitted the GHC version bump for the
`build_and_test` target.

Author: DigitalBrains1

.github/workflows/ci.yml

@@ -105,8 +105,8 @@ jobs:
             ghc: 8.8.4
             multiple_hidden: yes
 
-          - name: GHC 8.10.2, Multiple Hidden
-            ghc: 8.10.2
+          - name: GHC 8.10.7, Multiple Hidden
+            ghc: 8.10.7
             multiple_hidden: yes
 
           - name: GHC 9.0.2, Multiple Hidden