EndpointSlices were created in provision(), which was called only if certain
fields on the ExternalName Service had changed. If an EndpointSlice was
deleted, it was never re-created (because the owning Service had not
changed).
Move EndpointSlice provisioning after this gated provision step so that it
runs on every reconcile.
Fixes#20322
Change-Id: I416fb5e4b40f2029efb97aa6ca7ceb3e31b0d52d
Signed-off-by: Becky Pauley <becky@tailscale.com>