fix(google-maps): fix update schematic

[terencehonles]

This change fixes the updateToV20 schematic which should be a factory function not the schematic itself.

Fixes the error:

❯ Updates the Angular Google Maps package to v20.                                                                                                      
✖ Migration failed: rule is not a function                                                                                                             
  See "/tmp/ng-1KmdDK/angular-errors.log" for further details.     

[crisbeto]

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

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