Set -source 8 in CMAKE_JAVA_COMPILE_FLAGS (#11385)

Summary:
build-windows-vs2022 jobs (e.g., https://app.circleci.com/pipelines/github/facebook/rocksdb/26641/workflows/7d1c58b8-7dd6-4dd6-a222-ecdfb0892c3b/jobs/593583) began failing with:

```
       (CustomBuild target) ->
         CUSTOMBUILD : error : Source option 7 is no longer supported. Use 8 or later. [C:\Users\circleci.PACKER-64370BA5\project\build\java\rocksdbjni_classes.vcxproj]
```

So, this PR tries setting `-source 8` instead.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11385

Reviewed By: ltamasi

Differential Revision: D45058172

Pulled By: ajkr

fbshipit-source-id: b0daa1ea6f576c8417add40bd6c92710d329c44d

java/CMakeLists.txt

@@ -4,7 +4,7 @@ if(${CMAKE_VERSION} VERSION_LESS "3.11.4")
     message("Please consider switching to CMake 3.11.4 or newer")
 endif()
 
-set(CMAKE_JAVA_COMPILE_FLAGS -source 7)
+set(CMAKE_JAVA_COMPILE_FLAGS -source 8)
 
 set(JNI_NATIVE_SOURCES
         rocksjni/backup_engine_options.cc