Ignore stylelint false positive in datepicker overrides

We can't simply remove the nesting selector in this case since it's
inside a `:not` pseudo-class. Removing the nesting selector would cause
a syntax error.

.stylelintrc.yml

@@ -90,6 +90,9 @@ rules:
   "@stylistic/number-leading-zero": "always"
   "@stylistic/selector-list-comma-newline-after": always
 overrides:
+  - files: app/assets/stylesheets/datepicker_overrides.scss
+    rules:
+      scss/selector-no-redundant-nesting-selector: null
   - files: app/assets/stylesheets/legislation_process.scss
     rules:
       max-nesting-depth: 8