[ISSUE #3684] change client jdk version to 1.6

[duhenglucky]

Make sure set the target branch to develop

What is the purpose of the change

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Brief changelog

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Verifying this change

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[coveralls]

Coverage decreased (-0.2%) to 53.207% when pulling 1be3cc3 on duhenglucky:develop_jdk into 9f9b109 on apache:develop.

[codecov-commenter]

Codecov Report

Merging #3683 (1be3cc3) into develop (9f9b109) will decrease coverage by 0.14%.
The diff coverage is 19.44%.

@@              Coverage Diff              @@
##             develop    #3683      +/-   ##
=============================================
- Coverage      47.37%   47.23%   -0.15%     
+ Complexity      5044     5030      -14     
=============================================
  Files            627      627              
  Lines          41348    41371      +23     
  Branches        5372     5374       +2     
=============================================
- Hits           19588    19540      -48     
- Misses         19332    19406      +74     
+ Partials        2428     2425       -3     

Impacted Files	Coverage Δ
...rocketmq/client/impl/factory/MQClientInstance.java	52.96% <0.00%> (-0.33%)	⬇️
...mq/client/impl/producer/DefaultMQProducerImpl.java	44.98% <0.00%> (-0.97%)	⬇️
...g/apache/rocketmq/client/impl/MQClientAPIImpl.java	12.84% <50.00%> (-0.03%)	⬇️
...ent/impl/consumer/DefaultLitePullConsumerImpl.java	67.99% <100.00%> (-0.52%)	⬇️
.../rocketmq/client/producer/RequestFutureHolder.java	81.39% <100.00%> (ø)
.../trace/hook/ConsumeMessageOpenTracingHookImpl.java	74.35% <100.00%> (ø)
...rg/apache/rocketmq/common/stats/StatsSnapshot.java	84.61% <0.00%> (-15.39%)	⬇️
...tmq/logappender/log4j2/RocketmqLog4j2Appender.java	35.00% <0.00%> (-10.00%)	⬇️
.../apache/rocketmq/common/stats/MomentStatsItem.java	38.09% <0.00%> (-9.53%)	⬇️
...ache/rocketmq/common/stats/MomentStatsItemSet.java	39.13% <0.00%> (-8.70%)	⬇️
... and 14 more

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[vongosling]

What is this? For traditional cloud customers, you could create a release branch. Or say, you cancel JDK8 some special semantics, release two packages, one of them is backporting 1.6.

[dongeforever]

If there is no other significant reason, it is better to keep 1.6 for the client, to be compatible with traditional enterprise users.

[vongosling]

You can calculate the percentage of traditional enterprises in your customers？ If you could learn more life-cycle governance about production, you could not insist on this solution. I have given you another backport way to support and educate enterprise customers. I think we have many of these customers in ours. Pls remember, Open source doesn't kidnap by our business. You can support innovation and smooth upgrades in a more collaborative way.