[libc++][Github] Move back to main runner set (#171771)

This moves us back to the main runner set so that the next runner set
can be used for upgrading again when we want to do that. This also
captures the Github Runner version upgrade.

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

@@ -36,7 +36,7 @@ concurrency:
 jobs:
   stage1:
     if: github.repository_owner == 'llvm'
-    runs-on: llvm-premerge-libcxx-next-runners
+    runs-on: llvm-premerge-libcxx-runners
     continue-on-error: false
     strategy:
       fail-fast: false
@@ -73,7 +73,7 @@ jobs:
             **/crash_diagnostics/*
   stage2:
     if: github.repository_owner == 'llvm'
-    runs-on: llvm-premerge-libcxx-next-runners
+    runs-on: llvm-premerge-libcxx-runners
     needs: [ stage1 ]
     continue-on-error: false
     strategy:
@@ -148,19 +148,19 @@ jobs:
           'generic-static',
           'bootstrapping-build'
         ]
-        machine: [ 'llvm-premerge-libcxx-next-runners' ]
+        machine: [ 'llvm-premerge-libcxx-runners' ]
         include:
         - config: 'generic-cxx26'
-          machine: llvm-premerge-libcxx-next-runners
+          machine: llvm-premerge-libcxx-runners
         - config: 'generic-asan'
-          machine: llvm-premerge-libcxx-next-runners
+          machine: llvm-premerge-libcxx-runners
         - config: 'generic-tsan'
-          machine: llvm-premerge-libcxx-next-runners
+          machine: llvm-premerge-libcxx-runners
         - config: 'generic-ubsan'
-          machine: llvm-premerge-libcxx-next-runners
+          machine: llvm-premerge-libcxx-runners
         # Use a larger machine for MSAN to avoid timeout and memory allocation issues.
         - config: 'generic-msan'
-          machine: llvm-premerge-libcxx-next-runners
+          machine: llvm-premerge-libcxx-runners
     runs-on: ${{ matrix.machine }}
     steps:
       - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1