workflows: Stop building for Steam Runtime 3 on arm64

The experimental Steam Linux Runtime 3.0 (arm64) container is being
phased out, so games that want native arm64 binaries should upgrade to
Steam Linux Runtime 4.0.

steamrt/tasks#1032

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit d834351d69c8567d09c0a036d48f4b6661ab5dc3)

.github/workflows/create-test-plan.py

@@ -118,7 +118,6 @@ JOB_SPECS = {
     "ubuntu-latest": JobSpec(name="Ubuntu (latest)",                        os=JobOs.UbuntuLatest,      platform=SdlPlatform.Linux,       artifact="SDL-ubuntu-latest", ),
     "ubuntu-24.04-arm64": JobSpec(name="Ubuntu 24.04 (ARM64)",              os=JobOs.Ubuntu24_04_arm,   platform=SdlPlatform.Linux,       artifact="SDL-ubuntu24.04-arm64", ),
     "steamrt3": JobSpec(name="Steam Linux Runtime 3.0 (x86_64)",            os=JobOs.UbuntuLatest,      platform=SdlPlatform.Linux,       artifact="SDL-steamrt3",           container="registry.gitlab.steamos.cloud/steamrt/sniper/sdk:latest" ),
-    "steamrt3-arm64": JobSpec(name="Steam Linux Runtime 3.0 (arm64)",       os=JobOs.Ubuntu24_04_arm,   platform=SdlPlatform.Linux,       artifact="SDL-steamrt3-arm64",     container="registry.gitlab.steamos.cloud/steamrt/sniper/sdk/arm64:latest" ),
     "steamrt4": JobSpec(name="Steam Linux Runtime 4.0 (x86_64)",            os=JobOs.UbuntuLatest,      platform=SdlPlatform.Linux,       artifact="SDL-steamrt4",           container="registry.gitlab.steamos.cloud/steamrt/steamrt4/sdk:latest", more_hard_deps = True, ),
     "steamrt4-arm64": JobSpec(name="Steam Linux Runtime 4.0 (arm64)",       os=JobOs.Ubuntu24_04_arm,   platform=SdlPlatform.Linux,       artifact="SDL-steamrt4-arm64",     container="registry.gitlab.steamos.cloud/steamrt/steamrt4/sdk/arm64:latest", more_hard_deps = True, ),
     "ubuntu-intel-icx": JobSpec(name="Ubuntu 22.04 (Intel oneAPI)",         os=JobOs.Ubuntu22_04,       platform=SdlPlatform.Linux,       artifact="SDL-ubuntu22.04-oneapi", intel=IntelCompiler.Icx, ),