Rename Template types to Preset types with backward-compatible API support

This MR completes the migration from the old "Template" terminology to the new "Preset" terminology, while ensuring full backward compatibility for existing clusters.

To make the transition smoother, we should first introduce the new SylvaUnitsReleasePreset type while keeping the existing SylvaUnitsReleaseTemplate type intact. This allows us to propagate the updated operator and CRDs safely. Subsequent MRs can then progressively adopt the new Preset types without blocking any ongoing or parallel work. Only after this rollout will we remove the old SylvaUnitsReleaseTemplate type in a later update of the operator. This will ensure a non-breaking transition

closes #80 (closed)