docs(sample): use Java 8 for native image sample

[mpeddada1]

The native image sample is failing during the nightly build with the following error:

ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] javac: invalid target release: 11
Usage: javac <options> <source files>
use -help for a list of possible options

This is because the Kokoro job uses Java 8. Setting the compiler Java version of the sample to Java 8 should be able to address this issue.

[anguillanneuf]

Thank you!