Previously, seamless key renewal was an opt-in feature. Customers had
to set a `seamless-key-renewal` node attribute in their policy file.
This patch enables seamless key renewal by default for all clients.
It includes a `disable-seamless-key-renewal` node attribute we can set
in Control, so we can manage the rollout and disable the feature for
clients with known bugs. This new attribute makes the feature opt-out.
Updates tailscale/corp#31479
Signed-off-by: Alex Chan <alexc@tailscale.com>