Update release schedule for April 2026 patches

[Caesarsage]

• Promotes 1.33.11, 1.34.7, and 1.35.4 from next into previousPatches (April 2026 patch releases shipped on schedule).
• Sets next to 1.33.12 / 1.34.8 / 1.35.5 with the May 2026 cherry-pick deadlines.
• Refreshes upcoming_releases (drops the April row, adds July).
• Removes a stray 1.34.0 entry from 1.34's previousPatches. The .0 of a minor isn't a patch and the other minors (1.33, 1.35, 1.36) follow the convention of starting previousPatches at .1. schedule-builder does not produce .0 entries.

Generated by running schedule-builder -uc data/releases/schedule.yaml -e data/releases/eol.yaml (per the file header), then manually removing the 1.34.0 block.

Closes #55542

[k8s-ci-robot]

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[Caesarsage]

closes #55607