tests/e2e: drop the deprecated KOPS_CONTROL_PLANE_SIZE fallback - #18682

KOPS_CONTROL_PLANE_COUNT has been the supported name since #15628 (July 2023). Since then tests/e2e/scenarios/lib/common.sh and tests/e2e/scenarios/lib/upgrade.sh have each carried an identical shim that exists only to warn and reassign:

# TODO: Switch scripts to use KOPS_CONTROL_PLANE_COUNT
if [[ -n "${KOPS_CONTROL_PLANE_SIZE:-}" ]]; then
    echo "Recognized (deprecated) KOPS_CONTROL_PLANE_SIZE=${KOPS_CONTROL_PLANE_SIZE}, please set KOPS_CONTROL_PLANE_COUNT instead"
    KOPS_CONTROL_PLANE_COUNT=${KOPS_CONTROL_PLANE_SIZE}
fi

The only callers were the prow jobs. kubernetes/test-infra#37674 switches all 30 of them to KOPS_CONTROL_PLANE_COUNT, which resolves the TODO and makes both shims dead.

The --control-plane-count / --control-plane-size flags themselves are unchanged, as is the ${KOPS_CONTROL_PLANE_COUNT:-1} default at each call site. KOPS_CONTROL_PLANE_SIZE no longer appears anywhere in the repo after this.

Ordering — this one is not order-independent

Hold until kubernetes/test-infra#37674 merges. Unlike the karpenter pair (#18681 / kubernetes/test-infra#37675), the failure mode here is silent rather than inert: a job that still sets only KOPS_CONTROL_PLANE_SIZE falls through to the :-1 default and creates a single control plane node instead of three. The affected jobs are the HA upgrade tests, which would keep passing while no longer testing what they are meant to test.

That also applies to this PR's own presubmits. pull-kops-aws-upgrade-* and the -upgrades-dns-none presubmits set KOPS_CONTROL_PLANE_SIZE today, so until #37674 lands they will exercise this branch with one control plane node. Green presubmits here are not evidence that the ordering is safe.

/hold

Testing

• bash -n on both modified scripts.
• Verified KOPS_CONTROL_PLANE_SIZE has zero remaining references repo-wide, and that both ${KOPS_CONTROL_PLANE_COUNT:-1} call sites are untouched.

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