feat(@angular/build): rename experimentalPlatform to platform in application builder

[alan-agius4]

The experimentalPlatform option within the ssr option of the application builder has been renamed to platform. This change promotes the feature from experimental status while maintaining its functionality for specifying the target server platform.

A migration has been added to automatically update existing configuration to the new option name.

[gemini-code-assist]

Code Review

This pull request promotes the experimentalPlatform option to platform in the application builder, moving it out of experimental status. The changes are well-contained, primarily involving renaming across the codebase. The inclusion of a migration script to automatically update existing projects is a thoughtful addition, and its implementation and tests appear solid. I've identified one minor issue in the schema.json where the description for the new platform option still contains experimental wording. A suggestion to update the description has been provided. Overall, this is a good and clear update.

[alan-agius4]

This PR was merged into the repository. The changes were merged into the following branches:

• main: af2c7e9

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