bump to 2.0.0 and drop java 1.7 support

[nobodyiam]

What's the purpose of this PR

bump to 2.0.0 and drop java 1.7 support

Which issue(s) this PR fixes:

see discussion #3896

Brief changelog

• update version to 2.0.0
• remove java.version override of apollo-client and apollo-core
• remove github action for java 1.7

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 #4015 (a583aa6) into master (67a78ef) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4015      +/-   ##
============================================
- Coverage     51.54%   51.49%   -0.06%     
+ Complexity     2533     2530       -3     
============================================
  Files           484      484              
  Lines         14839    14839              
  Branches       1537     1537              
============================================
- Hits           7649     7641       -8     
- Misses         6658     6666       +8     
  Partials        532      532              

Impacted Files	Coverage Δ
...work/apollo/biz/message/DatabaseMessageSender.java	50.00% <0.00%> (-14.59%)	⬇️
.../apollo/internals/RemoteConfigLongPollService.java	77.71% <0.00%> (-0.61%)	⬇️

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[Anilople]

Property <java.version>1.7</java.version> In module apollo-demo and apollo-mockserver should also delete?

apollo/apollo-demo/pom.xml

Line 30 in 8484178

<java.version>1.7</java.version>

apollo/apollo-mockserver/pom.xml

Line 32 in 8484178

<java.version>1.7</java.version>

[nobodyiam]

@Anilople Thanks for the reminder, I missed these places.