Upgrade postgres-prisma example to prisma 7

[AmanVarshney01]

Description

• Upgrades to prisma 7
• adds prisma.config.ts for migrations, schema, database url configurations
• use @prisma/adapter-pg driver adapter to query with postgres

Type of Change

• New Example
• Example updates (Bug fixes, new features, etc.)
• Other (changes to the codebase, but not to examples)

[vercel]

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