ci: Stop using self-hosted Linux/arm64 runners

These are no longer required since GitHub launched their own Linux/arm64 runners.

See https://github.blog/changelog/2025-01-16-linux-arm64-hosted-runners-now-available-for-free-in-public-repositories-public-preview/

build-matrix.json

@@ -13,6 +13,11 @@
       "os_name": "linux",
       "target_arch": "x64"
     },
+    {
+      "os": "ubuntu-24.04-arm",
+      "os_name": "linux",
+      "target_arch": "arm64"
+    },
     {
       "comment": "Explicit macOS version 13 is required for explicit x64 CPU.",
       "os": "macos-13",
@@ -32,12 +37,7 @@
     }
   ],
 
-  "comment2": "runners hosted by the owner, enabled by the ENABLE_SELF_HOSTED variable being set on the repo",
+  "comment2": "Self-hosted runners are not used since the introduction of GitHub-hosted Linux arm64 runners.  The feature still exists if a new platform becomes necessary.",
   "selfHosted": [
-    {
-      "os": "self-hosted-linux-arm64",
-      "os_name": "linux",
-      "target_arch": "arm64"
-    }
   ]
 }