Update Dubbo version to 2.6.x in sentinel-dubbo-adapter and demo

[sczyh30]

Describe what this PR does / why we need it

Update Dubbo version to 2.6.x in sentinel-dubbo-adapter and demo, since Dubbo 2.5.x will be end-of-life soon.

Does this pull request fix one issue?

NONE

Describe how you did it

Simply update the Dubbo version to 2.6.6.

Describe how to verify it

Run the demo.

Special notes for reviews

Plz also test for using Dubbo 2.5.x with the adapter.

[codecov-io]

Codecov Report

Merging #617 into master will increase coverage by 0.48%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #617      +/-   ##
============================================
+ Coverage     39.83%   40.31%   +0.48%     
- Complexity     1274     1288      +14     
============================================
  Files           278      278              
  Lines          8832     8832              
  Branches       1183     1183              
============================================
+ Hits           3518     3561      +43     
+ Misses         4869     4812      -57     
- Partials        445      459      +14

Impacted Files	Coverage Δ	Complexity Δ
...a/csp/sentinel/slots/statistic/base/LeapArray.java	70.58% <0%> (-1.97%)	34% <0%> (-1%)
...l/cluster/server/connection/ConnectionManager.java	76% <0%> (+2%)	11% <0%> (+1%)	⬆️
...a/com/alibaba/csp/sentinel/eagleeye/StatEntry.java	20.21% <0%> (+2.12%)	8% <0%> (+1%)	⬆️
...alibaba/csp/sentinel/eagleeye/StatRollingData.java	73.52% <0%> (+2.94%)	9% <0%> (+1%)	⬆️
...a/csp/sentinel/slots/statistic/base/LongAdder.java	34.04% <0%> (+17.02%)	11% <0%> (+7%)	⬆️
...a/csp/sentinel/slots/statistic/base/Striped64.java	60.41% <0%> (+34.37%)	10% <0%> (+5%)	⬆️

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[CarpenterLee]

LGTM