Validate isOutdated against all namespaces

This commit makes isOutdated validate a nodes necessity to update
against all namespaces, and not just the nodes own namespace (which made
more sense before).

getLastStateChange is now uses the passed namespaces as a filter,
meaning that not requesting any namespace will give you the total last
updated state.

In addition, the sync.Map is exchanged for a variant that uses generics
which allows us to remove some casting logic.

flake.nix

@@ -24,7 +24,7 @@
 
               # When updating go.mod or go.sum, a new sha will need to be calculated,
               # update this if you have a mismatch after doing a change to thos files.
-              vendorSha256 = "sha256-e3s10NsadWTEk7H7SbEvePhihJ1x2dS9UfIqPNIbJqI=";
+              vendorSha256 = "sha256-b6qPOO/NmcXsAsSRWZlYXZKyRAF++DsL4TEZzRhQhME=";
 
               ldflags = [ "-s" "-w" "-X github.com/juanfont/headscale/cmd/headscale/cli.Version=v${version}" ];
             };