Document & Reduce Minimum Required Java Version

[Valkryst]

Describe the problem you'd like to have solved

I'm currently evaluating a number of solutions (OpenFGA, Ory Keto, Permify, etc...) to be used in building a new permissions system for a large enterprise client.

As a part of the evaluation, I'm looking to ensure that the chosen solution has support for Java 8. The client is unable to update to a newer JDK, at least for now.

I didn't see the minimum required JDK version listed in the readme, or in the release notes. After looking through the Gradle-related files, it looks like the minimum required version is Java 11 and that it has been set to this since the first commit.

Describe the ideal solution

1. For the readme to be updated to include the minimum required Java version. A good spot may be near the line "It currently supports PostgreSQL 14 and MySQL 8.".
2. If possible, for the minimum required Java version to be set to Java 8.

[TomerPacific]

@rhamzeh = Can I help with this one?

[aaguiarz]

@TomerPacific I'm assigning it to you, thanks!

[TomerPacific]

@aaguiarz - Thanks, per @Valkryst's comment, should I just document the minimum supported version or also downgrade to Java 8 and see if everything works?

[TomerPacific]

@aaguiarz - Can you give the PR a look?
#122