upgrade spring boot to 2.6.6 and spring cloud to 2021.0.1

[nobodyiam]

What's the purpose of this PR

upgrade spring boot to 2.6.6 and spring cloud to 2021.0.1

Which issue(s) this PR fixes:

Fixes #4282

Brief changelog

• upgrade spring boot to 2.6.6
• spring cloud to 2021.0.1
• set spring.jpa.defer-datasource-initialization to true for tests
• fix mockito deprecated classes

Follow this checklist to help us incorporate your contribution quickly and easily:

• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit tests to verify the code.
• Run mvn clean test to make sure this pull request doesn't break anything.
• Update the CHANGES log.

[codecov-commenter]

Codecov Report

Merging #4295 (499d325) into master (039801a) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4295      +/-   ##
============================================
- Coverage     53.15%   53.13%   -0.02%     
+ Complexity     2668     2667       -1     
============================================
  Files           488      488              
  Lines         15253    15253              
  Branches       1577     1577              
============================================
- Hits           8107     8105       -2     
- Misses         6591     6592       +1     
- Partials        555      556       +1     

Impacted Files	Coverage Δ
...rk/apollo/spring/property/SpringValueRegistry.java	83.78% <0.00%> (-5.41%)	⬇️
.../framework/apollo/spring/property/SpringValue.java	87.71% <0.00%> (-1.76%)	⬇️
.../apollo/internals/RemoteConfigLongPollService.java	78.31% <0.00%> (+0.60%)	⬆️

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[lepdou]

LGTM