feat(@schematics/angular): use signal in app component

[cexbrayat]

PR Checklist

Please check to confirm your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• [x] 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
• [x] 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 new behavior?

Now that signals are recommended, this updates the generated AppComponent to use a signal for the title field.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[cexbrayat]

@alan-agius4 @dgp1130 PR is ready for review with your feedback

[dgp1130]

Thanks for the contribution, apologies for the long delay on this!

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