[ISSUE #6861] Modify CURRENT_VERSION to 5.1.2

[RongtongJin]

Which Issue(s) This PR Fixes

Fixes #6861

Brief Description

Modify CURRENT_VERSION to 5.1.2

How Did You Test This Change?

[codecov-commenter]

Codecov Report

Merging #6862 (b01ca36) into develop (a325d14) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             develop    #6862      +/-   ##
=============================================
+ Coverage      42.64%   42.66%   +0.02%     
- Complexity      9086     9092       +6     
=============================================
  Files           1121     1121              
  Lines          79775    79775              
  Branches       10409    10409              
=============================================
+ Hits           34018    34038      +20     
+ Misses         41478    41461      -17     
+ Partials        4279     4276       -3     

Impacted Files	Coverage Δ
...ain/java/org/apache/rocketmq/common/MQVersion.java	99.83% <100.00%> (ø)

... and 16 files with indirect coverage changes

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