fixing javadoc to compatible with jdk8 source

[xiangfu0]

Description

Since we need to release several packages with jdk8, make plugin source to 8 to ensure javadoc is not complaining

Upgrade Notes

Does this PR prevent a zero down-time upgrade? (Assume upgrade order: Controller, Broker, Server, Minion)

• Yes (Please label as backward-incompat, and complete the section below on Release Notes)

Does this PR fix a zero-downtime upgrade introduced earlier?

• Yes (Please label this as backward-incompat, and complete the section below on Release Notes)

Does this PR otherwise need attention when creating release notes? Things to consider:

• New configuration options
• Deprecation of configurations
• Signature changes to public methods/interfaces
• New plugins added or old plugins removed

• Yes (Please label this PR as release-notes and complete the section on Release Notes)

Release Notes

Documentation

[codecov-commenter]

Codecov Report

Merging #7754 (30ef05c) into master (570e983) will increase coverage by 0.10%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #7754      +/-   ##
============================================
+ Coverage     71.53%   71.63%   +0.10%     
+ Complexity     4065     4063       -2     
============================================
  Files          1577     1577              
  Lines         80552    80597      +45     
  Branches      11972    11978       +6     
============================================
+ Hits          57621    57735     +114     
+ Misses        19051    18969      -82     
- Partials       3880     3893      +13     

Flag	Coverage Δ
integration1	29.42% <ø> (+0.26%)	⬆️
integration2	27.86% <ø> (+0.01%)	⬆️
unittests1	68.56% <ø> (-0.05%)	⬇️
unittests2	14.61% <ø> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...a/manager/realtime/RealtimeSegmentDataManager.java	50.00% <0.00%> (-25.00%)	⬇️
...he/pinot/segment/local/segment/store/IndexKey.java	65.00% <0.00%> (-5.00%)	⬇️
...ntroller/helix/core/minion/TaskMetricsEmitter.java	86.36% <0.00%> (-4.55%)	⬇️
...rg/apache/pinot/broker/routing/RoutingManager.java	87.09% <0.00%> (-2.16%)	⬇️
...core/startree/operator/StarTreeFilterOperator.java	86.52% <0.00%> (-2.13%)	⬇️
...e/pinot/segment/local/io/util/PinotDataBitSet.java	95.62% <0.00%> (-1.46%)	⬇️
...nMaxValueBasedSelectionOrderByCombineOperator.java	72.51% <0.00%> (-0.77%)	⬇️
...ache/pinot/common/metadata/ZKMetadataProvider.java	81.20% <0.00%> (-0.76%)	⬇️
...e/pinot/core/transport/InstanceRequestHandler.java	60.75% <0.00%> (-0.36%)	⬇️
.../aggregation/function/ModeAggregationFunction.java	88.37% <0.00%> (-0.28%)	⬇️
... and 26 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 570e983...30ef05c. Read the comment docs.