Feat: Update for optimize-css-assets-webpack-plugin 5.0 compatibility.

[x8x]

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

-> Update dependency

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:
While it is safe to update all dependencies listed in package.json, optimize-css-assets-webpack-plugin 5.0 depends on cssnano 4.x (before 3.x see NMFR/optimize-css-assets-webpack-plugin@62ea898#diff-b9cfc7f2cdf78a7f4b91a753d10865a2) that had a breaking change, see here for specific infos:
NMFR/optimize-css-assets-webpack-plugin#65 (comment)
Probably a good time to update all dependencies and test, so next branch can benefit form fresh dependencies.
Also see here for cssnano 4.x default transforms changes:
cssnano/cssnano#358
And support for passing options cssnano v4:
NMFR/optimize-css-assets-webpack-plugin/pull/73/files