util-docker,tests: Up clang support: >=v10

The compiler tests are failing to to a compile bug in Clang 7:
https://github.com/gem5/gem5/actions/runs/10170081794

Given Ubuntu 20.04 APT installs v10 by default (i.e., with `apt install
clang`). This is the oldest LTS Ubuntu version. It therefore seems
sensible to drop support for older (<v10) versions of clang.

Change-Id: I4c48223b80306422beac1464c09f03397c156ba1

util/dockerfiles/docker-bake.hcl

@@ -160,7 +160,7 @@ target "clang-compilers-base-20-04" {
   context = "ubuntu-20.04_clang-version"
   dockerfile = "Dockerfile"
   matrix = {
-    ver = ["7", "8", "9", "10", "11", "12"]
+    ver = ["10", "11", "12"]
   }
   args = {
     version = ver