Update PostgreSQL version from 14 to 16 for Heroku compatibility

[brantenK]

Heroku has deprecated PostgreSQL 14. Updated template to use PostgreSQL 16, which is supported. This fixes deployment failures with the error: 'Unsupported version: 14. We support the following versions: 15, 16, 17'

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Note

Update app.json to use Heroku Postgres addon version 16 instead of 14.

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