Bump Xcode to 12.5.1 and bump firebase-ios-sdk to 8.6.0

[uhooi]

Issues

• close #187
• close #194
• close #209

References

• https://github.com/uhooi/UhooiPicBook/pull/176
• https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idruns-on
• https://github.com/actions/virtual-environments/blob/main/images/macos/macos-11-Readme.md#xcode
• https://firebase.google.com/support/release-notes/ios

[codecov[bot]]

Codecov Report

Merging #188 (11dda2b) into develop (8e59a2d) will decrease coverage by 3.17%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #188      +/-   ##
===========================================
- Coverage    55.02%   51.85%   -3.18%     
===========================================
  Files           10       10              
  Lines          189      189              
  Branches        86       87       +1     
===========================================
- Hits           104       98       -6     
- Misses          85       91       +6     

Impacted Files	Coverage Δ
Shared/Util/ImageCacheManager.swift	12.00% <0.00%> (-24.00%)	:arrow_down:

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