Switch to GitHub Actions public ubuntu-24.04-arm runner (#321)

GitHub recently announced ARM support when using their
public runners:
https://github.blog/changelog/2025-01-16-linux-arm64-hosted-runners-now-available-for-free-in-public-repositories-public-preview/

This means we don't need to use the custom runner group
for ARM CI jobs any more, which have a slower boot time,
since they don't have a warm slack pool of machines.

For example the CI run in #320 has been waiting for 11 mins
for a runner to be available:

```
Waiting for a runner to pick up this job...
```

GUS-W-17659818.

.github/workflows/ci.yml

@@ -51,7 +51,7 @@ jobs:
             arch: "arm64"
           - builder: "builder:20"
             arch: "arm64"
-    runs-on: ${{ matrix.arch == 'arm64' && 'pub-hk-ubuntu-24.04-arm-medium' || 'ubuntu-24.04' }}
+    runs-on: ${{ matrix.arch == 'arm64' && 'ubuntu-24.04-arm' || 'ubuntu-24.04' }}
     env:
       INTEGRATION_TEST_BUILDER: heroku/${{ matrix.builder }}
     steps: