Refactor generator to output Angular 21 apps

[BenjaminPabst]

Our generator currently outputs Angular 19 apps. To make use of our new branching and release strategy (see linked story) we should create a new version of the generator that outputs a fully functional Angular 21 app. This task depends on #328.

• Use angular 20 generator to get an app
• Update it to angular 21
• Use @BenjaminPabst prompt to reintegrate the changes in the generator
• test
• commit+push+release

[rogocoffee]

suggestion from @markuczy

1. Review
2. Create a follow up task to check if generator app works with angular 21 shell

[BenjaminPabst]

@prathamMalavi We also want to rename the package nx-plugin to angular-generator:

• Project Folder
• NPM Package (package.json+package-lock)
• Other relevant locations (documentation for now as well, we will adapt once react is included)