fix(@schematics/angular): use protected for class member

[cexbrayat]

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 new behavior?

This follows the updated style guide recommendations.

https://next.angular.dev/style-guide#use-protected-on-class-members-that-are-only-used-by-a-components-template

Does this PR introduce a breaking change?

• Yes
• No

Other information

[alan-agius4]

The changes were merged into the following branches: main, 20.0.x

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