[tobixen]

Closes #585

The probes for the two new features are in a caldav-server-tester PR (to follow).

Commit message:

feat: compatibility features replace the old flags

Two old-style flags were still set by a server profile.  They become
compatibility features: save-load.todo.uncomplete (GMX keeps a task
completed) and search.time-range.todo.no-dtstart.no-due (Zimbra misses
a task with neither DTSTART nor DUE).  The latter follows no-dtstart
unless set, so DAViCal and Stalwart need no entry.  The other seven
flags were set by no profile and are deleted with the test branches
reading them, as are incompatibility_description and old_flags.

Prompt: Please work on the issue about finally getting rid of the old compatibility_hints [https://github.com/python-caldav/caldav/issues/585].  This involves some work in caldav-server-tester as well, as the policy is that all the "feature hints" should come with checks.
Followup-Prompt: "no-dates" doesn't sound right, it may apply to "something having time but not a date".  timeless?  no-dt?  ...todo.without-dt? [on the name proposed for the feature replacing vtodo_datesearch_notime_task_is_skipped]  It's not a breaking change, those compatibility_hints was (sic) never intended for anything else than disabling/enabling tests, it was never part of the public API.  Even the new featureset is explicitly declared to be unstable in 3.x.  Testing towards GMX worked earlier this week, the authorization error problem has been resolved.  The zimbra container is already running.  caldav-server-tester having red pull requests until caldav is released is normal.  Go ahead.

Assisted-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Reviewed-by: Tobias Brox <tobias@redpill-linpro.com>