Dependencies: relax icalendar to >6.0.0, require xandikos >=0.3.3

The icalendar >7.0.0 requirement was premature; no released 3.x code
actually requires icalendar 7 features.  Reverted to >6.0.0 to avoid
breaking environments that haven't upgraded yet.

xandikos >=0.3.3 is now required for tests: earlier versions lack the
rate-delay configuration fields exercised by the integration test suite.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: tobixen

pyproject.toml

@@ -56,7 +56,7 @@ dependencies = [
   "niquests",
   "recurring-ical-events>=2.0.0",
   "typing_extensions;python_version<'3.11'",
-  "icalendar>7.0.0",
+  "icalendar>6.0.0",
   "icalendar-searcher>=1.0.5,<2",
   "dnspython",
   "python-dateutil",
@@ -79,7 +79,7 @@ test = [
   "manuel",
   "proxy.py",
   "tzlocal",
-  "xandikos>=0.2.12",
+  "xandikos>=0.3.3",
   "radicale",
   "pyfakefs",
   "httpx",