[7.0] Adds upgrade guide and better feedback

[nunomaduro]

This Pull Request addresses the migration of Laravel Scout to Algolia PHP API Client v2:

• Adds an UPGRADE.md - Since I believe that there are developers relying on the behavior of the Algolia PHP Client V1 (c1f44c7)
• Better feedback when dependencies are not installed correctly (768ed1c). Here is a example on Laravel Nova:

• Re-adds user agent that got removed here (9e23cf3)

Here is the Pull Request that updates the documentation: laravel/docs#4964

[driesvints]

@nunomaduro awesome, thanks!