Deprecate Java 8

[brunoocasali]

Description
We should move on and deprecate Java 8 compatibility. This will make tremendous improvements in the way we maintain the SDK.

But I only think we should do it if we are sure that at least 80% or more of our users are using Java 9 and newer.

TODO:

• Ensure the Java version is collected in the analytics (headers)
• Generate reports and make a decision
• Plan the changes and the breaking changes
• Upgrade to Gradle v9 #869
• Find a good Java version to be the minimum required version that are still supported by a lot of users