Bump the pinned ical-proxy image from main-4783abe to main-73b5d97.
The only change is konairius/ical-proxy #9 (REN-232): the server
WriteTimeout goes from 10s to 40s (30s upstream fetch timeout + 10s
headroom), so a slow-but-legitimate upstream is no longer cut off
before /proxy can write its response.
Refs: REN-239, REN-232
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>