[SPE-2273] Java - Ensure Java 8 Compatibility

[TristanSpeakEasy]

Java version

Java support for LTS versions Java 8, 11, 17 extend to at least 2026.

Speakeasy-generator Java SDKs require Java 11+.

There is a competitive advantage in supporting Java 8+. openapi-generator failed to see this (I discussed it with them [here](OpenAPITools/openapi-generator#15553)) and by requiring Java 11+ they will lose or frustrate some users (especially enterprises with large Java 8 codebases). The only feature of Java 9+ that is potentially useful is the support for the HTTP PATCH verb in the reworked platform HTTP classes. As we use Apache Httpclient 4.x for HTTP interactions we can easily base our generated code on Java 8.

_{From SyncLinear.com | SPE-2273}