ObserverManager: Use ConcurrentDictionary

[ScarletKuro]

Description

Use ConcurrentDictionary instead of Dictionary.
We wanted to use it before, but had some issues with it: #6981 (comment)
This is a lot better than copying the list.
I also decided the logs to be Trace type, since most people have Debug as their minimum even in prod.

How Has This Been Tested?

Type of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or improvement to the website or code docs)

Checklist

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.00%. Comparing base (2e29d6d) to head (ed09c02).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev   #10023   +/-   ##
=======================================
  Coverage   90.99%   91.00%           
=======================================
  Files         409      409           
  Lines       12508    12508           
  Branches     2442     2442           
=======================================
+ Hits        11382    11383    +1     
  Misses        573      573           
+ Partials      553      552    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.