When a rotation signature chain reaches a certain size, remove the
oldest rotation signature from the chain before wrapping it in a new
rotation signature.
Since all previous rotation signatures are signed by the same wrapping
pubkey (node's own tailnet lock key), the node can re-construct the
chain, re-signing previous rotation signatures. This will satisfy the
existing certificate validation logic.
Updates #13185
Signed-off-by: Anton Tolchanov <anton@tailscale.com>
fd6686d81a