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/default.nix

@@ -155,8 +155,8 @@ in {
 
     featureGates = mkOption {
       description = "List set of feature gates.";
-      default = [];
-      type = types.listOf types.str;
+      default = {};
+      type = types.attrsOf types.bool;
     };
 
     masterAddress = mkOption {