Fix UPGRADE.md

[rob006]

• Fix indentation for Upgrade from Yii 2.0.50 section
• Move Upgrade from Yii 2.0.53 content to Upgrade from Yii 2.0.52 (these changes were introduced in 2.0.53)
• Fix deprecation notice

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.45%. Comparing base (6ee5d95) to head (c30f3be).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #20458      +/-   ##
============================================
- Coverage     64.47%   64.45%   -0.02%     
  Complexity    11573    11573              
============================================
  Files           433      433              
  Lines         37597    37597              
============================================
- Hits          24239    24234       -5     
- Misses        13358    13363       +5     

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[Arhell]

lgtm

[samdark]

Thank you!