The ADC server runs as a sidecar that outlives the controller process. Losing the
lease terminates the manager container but not the sidecar, so the baseline ADC
holds per cacheKey -- the last synced content plus the conf_version it generated --
survives a leadership change. Once the pod is elected again, ADC diffs against that
frozen snapshot and carries over conf_versions older than the ones the leader in
between has already pushed. APISIX standalone requires *_conf_version to be
monotonic and rejects the request:

    upstreams_conf_version must be greater than or equal to (1779434128737)

The data plane validates the configuration as a whole, so every later sync keeps
failing and no update lands at all, endpoint updates included. Backends running a
single pod then return 502/504 once that pod is rescheduled, while multi-pod
backends keep serving on their remaining endpoints.

Winning the election is the one moment a baseline from an earlier term can enter the
picture, so that is where to answer it: the provider, which only runs once elected,
puts every baseline in doubt, and the first sync of each cacheKey re-derives it from
the data plane through ADC's bypassCache before anything is pushed from it. Only a
sync ADC accepts settles the question, so a rebuild that never landed is attempted
again rather than assumed.

Prevention cannot cover a desync no leadership change explains -- another writer on
this data plane, something we did not foresee -- and the only way any of that shows
itself is the data plane refusing a conf_version. So that rejection rebuilds the
baseline and pushes once more. It is recognised by the field it names rather than by
the sentence around it: conf_version belongs to the standalone admin API, we send
those keys ourselves, while the prose around them is APISIX's to reword. This is a
net, not the fix: nothing else rebuilds on it, and the reported bug does not come
back if it ever stops firing.

bypassCache needs ADC >= 0.27.0, so bump ADC_VERSION to 0.27.1. The sidecar image
the chart deploys has to move with it, or the rebuild is answered with a schema
error instead of healing.

Fixes apache#2774