fix: use previous release tag for benchmark comparison instead of release branch

[bhope]

The ci-benchmark-tests-releasebranch job compares benchmarks against release branch by fetching it from upstream. This requires the release branch to exist on the upstream repo before the release PR can merge.

The CI job to always fail on release PRs. Example https://github.com/kubernetes/kube-state-metrics/actions/runs/25844106890/job/75935367272?pr=2949

Fix by switching to compare prior tags instead. Tags always exist on upstream, so this works in all cases.

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[bhope]

cc @rexagod @mrueg