Remove ubuntu:focal and debian:11 from arm64 test matrix

PowerShell 7.6.0 requires GLIBC_2.33+ which is unavailable on these
older platforms. ubuntu:focal is already obsolete and debian:11 reaches
EOL in Aug 2026.

Agent-Logs-Url: https://github.com/devcontainers/features/sessions/6a803744-ae1d-4c84-8e04-5bb2c0df1523

Co-authored-by: Kaniska244 <[email protected]>

Author: Copilot

.github/workflows/test-pr-arm64.yaml

@@ -33,9 +33,7 @@ jobs:
         features: ${{ fromJSON(needs.detect-changes.outputs.features) }}
         baseImage:
           [
-            "ubuntu:focal",
             "ubuntu:jammy",
-            "debian:11",
             "debian:12",
             "mcr.microsoft.com/devcontainers/base:ubuntu",
             "mcr.microsoft.com/devcontainers/base:debian",