Update to connectionName for database session and rename 'postgres' to 'pg'

[rayfadh]

🔗 Linked issue

N/A.

❓ Type of change

• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

connection is now connectionName based on DatabaseStoreConfig interface from @adonisjs/session package.
https://github.com/adonisjs/session/blob/7c2d88917c3ed678b982def80b9701d11fb0f866/src/types.ts#L291-L292

Also, renamed the connectionName example from 'postgres' to 'pg' since the stubs from the starter kits are set default to 'pg'.

api starter kit:
https://github.com/adonisjs/starter-kits/blob/ba295f476e1880b893d88952e625ab2d1d80bc98/api/config/database.ts#L51-L55

api-monorepo starter kit:
https://github.com/adonisjs/starter-kits/blob/ba295f476e1880b893d88952e625ab2d1d80bc98/api-monorepo/apps/backend/config/database.ts#L51-L55

inertia-react starter kit:
https://github.com/adonisjs/starter-kits/blob/ba295f476e1880b893d88952e625ab2d1d80bc98/inertia-react/config/database.ts#L42-L46

inertia-vue starter kit:
https://github.com/adonisjs/starter-kits/blob/ba295f476e1880b893d88952e625ab2d1d80bc98/inertia-vue/config/database.ts#L42-L46

hypermedia starter kit:
https://github.com/adonisjs/starter-kits/blob/ba295f476e1880b893d88952e625ab2d1d80bc98/hypermedia/config/database.ts#L35-L39

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.