New icicle template models are not prefixed with "template_"

Relates to MC-308226 and MC-308227.

26.4 snapshot 3 adds two template models for icicles (icicle.json and icicle_base.json). Despite being template models (having no textures of their own and only referenced as a mesh by other model files), the “template_” prefix is nowhere to be seen on them.

To fix: rename these to template_icicle.json and template_icicle_base.json