Move all libc++ builders to one machine type.

There are ongoing issues with the libc++ bots, some of them seem
related to a new release of the gha action runner controllers.

Until I get this figured out, it's a lot easier to have 99% of the
bots using a single machine shape

.github/workflows/libcxx-build-and-test.yaml

@@ -44,7 +44,7 @@ env:
 jobs:
   stage1:
     runs-on:
-      group: libcxx-runners-16
+      group: libcxx-runners-8
     continue-on-error: false
     strategy:
       fail-fast: true
@@ -163,10 +163,10 @@ jobs:
           machine: libcxx-runners-8
           std_modules: 'ON'
         - config: 'generic-asan'
-          machine: libcxx-runners-16
+          machine: libcxx-runners-8
           std_modules: 'OFF'
         - config: 'generic-tsan'
-          machine: libcxx-runners-16
+          machine: libcxx-runners-8
           std_modules: 'OFF'
         - config: 'generic-ubsan'
           machine: libcxx-runners-8