PR Checklist

Please check to confirm your PR fulfills the following requirements:

• The commit message follows our guidelines:https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

executor:@angular-devkit/build-angular:dev-server

Users can use customized postcss config, butjson only, have a lot of limitions.

Issue Number:#27819

What is the new behavior?

Users can usejs config file(.js,.cjs and.mjs), it not only can handle all situations ofjson config, but also more powerful.

And thepostcss early migration guide still recommend to usejs as configuration file.

Does this PR introduce a breaking change?

• Yes
• No

Not supportjson configuration again. Try supportjson orts will increase the complexity of code bases.

Other information