fix compatibility heatmap with mysql 8

[gaydin]

Updated SQL query for compatibility with the latest version of mysql.

Resolves: #5220

[codecov-io]

Codecov Report

Merging #5232 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5232      +/-   ##
==========================================
+ Coverage   37.47%   37.48%   +0.01%     
==========================================
  Files         310      310              
  Lines       45923    45923              
==========================================
+ Hits        17210    17215       +5     
+ Misses      26238    26234       -4     
+ Partials     2475     2474       -1

Impacted Files	Coverage Δ
models/user_heatmap.go	80% <100%> (ø)	⬆️
models/repo_list.go	64.55% <0%> (+1.26%)	⬆️
modules/process/manager.go	81.15% <0%> (+4.34%)	⬆️

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