Copyright notices in software are not supposed to update
the year in the header.
Because we have a CI check for `go generate`, we're failing
CI until we go update all of the copyright headers in
generated files to say 2023.
Instead, relax the requirement to always have a year in the
copyright header.
Fixes https://github.com/tailscale/tailscale/issues/6865
Signed-off-by: Denton Gentry <dgentry@tailscale.com>