package.json: Bump clean-css to current version

The CLI tool (the only thing that we actually use directly) got split
off into a separate NPM module. Also, `--skip-import` was replaced with
the more general `--inline`.

This fixes a vulnerability scan warning:
https://npmjs.com/advisories/785

package.json

@@ -46,7 +46,7 @@
     "babel-eslint": "^10.0.3",
     "babel-loader": "^8.0.4",
     "chrome-remote-interface": "^0.28.0",
-    "clean-css": "~3.4.20",
+    "clean-css-cli": "^4.2.1",
     "copy-webpack-plugin": "^4.5.2",
     "css-loader": "^3.2.0",
     "eslint": "^6.2.2",

src/base1/Makefile.am

@@ -25,7 +25,7 @@ AM_TESTS_ENVIRONMENT = export G_DEBUG=fatal-criticals,fatal-warnings ; export XD
 
 dist/base1/cockpit.min.css: src/base1/cockpit.css
 	$(AM_V_GEN) $(MKDIR_P) $(dir $@) && \
-	$(srcdir)/tools/missing $(CLEAN_CSS) --keep-line-breaks --output=$@ --skip-import --source-map --source-map-inline-sources $^
+	$(srcdir)/tools/missing $(CLEAN_CSS) --keep-line-breaks --output=$@ --inline none --source-map --source-map-inline-sources $^
 dist/base1/patternfly.min.css: dist/base1/patternfly-base.css dist/base1/patternfly-additions.css
 	$(MIN_CSS_RULE)
 dist/base1/jquery.js: