nixos/kubernetes: refactor feature gates to attrsOf bool, making it possible to disable featureGates

This is a breaking change, requiring users of `featureGates` to change
from a `listOf str` to `attrsOf bool`.

Before:
```nix
featureGates = [ "EphemeralContainers" ];
extraOpts = pkgs.lib.concatStringsSep " " (
[
  "--container-runtime=remote"
  ''--feature-gates="CSIMigration=false"''
});
```

After:
```nix
featureGates = {EphemeralContainers = true; CSIMigration=false;};
```

This is much nicer, and sets us up for later work of migrating to
configuration files for other services, like e.g. has been happening
with kubelet (see: #290119).

Signed-off-by: Christina Sørensen <christina@cafkafk.com>

nixos/modules/services/cluster/kubernetes/kubelet.nix

@@ -65,7 +65,7 @@ let
     // lib.optionalAttrs (cfg.tlsKeyFile != null)   { tlsPrivateKeyFile = cfg.tlsKeyFile; }
     // lib.optionalAttrs (cfg.clusterDomain != "")  { clusterDomain = cfg.clusterDomain; }
     // lib.optionalAttrs (cfg.clusterDns != "")     { clusterDNS = [ cfg.clusterDns ] ; }
-    // lib.optionalAttrs (cfg.featureGates != [])   { featureGates = cfg.featureGates; }
+    // lib.optionalAttrs (cfg.featureGates != {})   { featureGates = cfg.featureGates; }
   ));
 
   manifestPath = "kubernetes/manifests";
@@ -185,10 +185,10 @@ in
     };
 
     featureGates = mkOption {
-      description = "List set of feature gates";
+      description = "Attribute set of feature gate";
       default = top.featureGates;
       defaultText = literalExpression "config.${otop.featureGates}";
-      type = listOf str;
+      type = attrsOf bool;
     };
 
     healthz = {