Prisma Schema Docs: make `postgresqlExtensions` GA

[Jolg42]

Part of prisma/orm#19381

Mentioned in

• https://github.com/prisma/docs/blob/main/content/200-concepts/100-components/01-prisma-schema/10-postgresql-extensions.mdx?plain=1
• https://github.com/prisma/docs/blob/main/content/400-reference/200-api-reference/100-prisma-schema-reference.mdx?plain=1#L3292
• https://github.com/prisma/docs/blob/main/content/300-guides/025-migrate/100-developing-with-prisma-migrate/165-enable-native-database-functions.mdx?plain=1#L18
• https://github.com/prisma/docs/blob/main/content/200-concepts/100-components/01-prisma-schema/06-features-without-psl-equivalent.mdx?plain=1#L53
• https://github.com/prisma/docs/blob/main/content/200-concepts/100-components/250-preview-features/050-client-preview-features.mdx?plain=1#L29

[Jolg42]

Closing this for now, as this is not part of Prisma 5 anymore.