add disclaimer and remove outdated CLA reference

[hubgeter]

This project has just entered the Apache Incubator. As required by the Incubator policy,
added the standard disclaimer to clarify the project's incubation status.
Also removed references to the previously used CCLA, which is no longer applicable
under the Apache Software Foundation's governance.

[popduke]

Thanks buddy.

[coveralls]

Pull Request Test Coverage Report for Build 15184105174

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 45 unchanged lines in 10 files lost coverage.
• Overall coverage decreased (-0.06%) to 62.632%

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Files with Coverage Reduction	New Missed Lines	%
com/baidu/bifromq/basecluster/transport/TCPTransport.java	1	89.05%
com/baidu/bifromq/basecrdt/store/AntiEntropy.java	1	83.16%
com/baidu/bifromq/basekv/store/wal/KVRangeWAL.java	1	93.33%
com/baidu/bifromq/basekv/raft/RaftNodeStateLeader.java	2	85.23%
com/baidu/bifromq/basehlc/HLC.java	3	67.74%
com/baidu/bifromq/dist/trie/TopicFilterIterator.java	3	73.66%
com/baidu/bifromq/basecluster/memberlist/AutoDropper.java	4	80.62%
com/baidu/bifromq/basekv/raft/RaftNodeStateFollower.java	8	80.75%
com/baidu/bifromq/mqtt/handler/MQTTSessionHandler.java	9	74.66%
com/baidu/bifromq/basekv/store/range/KVRangeFSM.java	13	74.36%

Totals
Change from base Build 15062003270:	-0.06%
Covered Lines:	23271
Relevant Lines:	35899

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💛 - Coveralls