build(deps): bump csstype from 2.6.8 to 2.6.9 (apache-superset#374)

Bumps [csstype](https://github.com/frenic/csstype) from 2.6.8 to 2.6.9.
- [Release notes](https://github.com/frenic/csstype/releases)
- [Commits](frenic/csstype@v2.6.8...v2.6.9)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

plugins/superset-ui-plugins/yarn.lock

@@ -7500,9 +7500,9 @@ cssstyle@^2.0.0:
     cssom "~0.3.6"
 
 csstype@^2.2.0, csstype@^2.5.7, csstype@^2.6.3, csstype@^2.6.4, csstype@^2.6.7, csstype@^2.6.8:
-  version "2.6.8"
-  resolved "https://registry.yarnpkg.com/csstype/-/csstype-2.6.8.tgz#0fb6fc2417ffd2816a418c9336da74d7f07db431"
-  integrity sha512-msVS9qTuMT5zwAGCVm4mxfrZ18BNc6Csd0oJAtiFMZ1FAx1CCvy2+5MDmYoix63LM/6NDbNtodCiGYGmFgO0dA==
+  version "2.6.9"
+  resolved "https://registry.yarnpkg.com/csstype/-/csstype-2.6.9.tgz#05141d0cd557a56b8891394c1911c40c8a98d098"
+  integrity sha512-xz39Sb4+OaTsULgUERcCk+TJj8ylkL4aSVDQiX/ksxbELSqwkgt4d4RD7fovIdgJGSuNYqwZEiVjYY5l0ask+Q==
 
 currently-unhandled@^0.4.1:
   version "0.4.1"