docs(prisma): Remove driverAdapters preview feature flag

[kinjo1130]

Prisma Documentation Update

Summary

Updates Prisma documentation to reflect that the driverAdapters preview feature flag is no longer required in Prisma 6.16.0 and later when using Turso with @prisma/adapter-libsql.

Changes

• Removed previewFeatures = ["driverAdapters"] from the example Prisma schema
• Updated the step title from "Enable the driverAdapters preview feature flag" to "Configure your Prisma schema"
• Added a note explaining that this flag was required before Prisma 6.16.0, but was promoted to General Availability in that version

Context

The driverAdapters feature was promoted from preview to General Availability in Prisma 6.16.0, so it no longer needs to be explicitly enabled as a preview feature. This change ensures the documentation reflects current best practices and doesn't require users to add unnecessary configuration flags.

References

• [Prisma 6.16.0 Release](https://github.com/prisma/prisma/releases/tag/6.16.0) - Driver Adapters GA announcement
• [Database Drivers Documentation](https://www.prisma.io/docs/orm/overview/databases/database-drivers)