Update dependency System.Reactive to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
System.Reactive	nuget	major	5.0.0 -> 6.0.0

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Configuration

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[codecov]

Codecov Report

Merging #975 (498ed3e) into master (09c69fb) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #975      +/-   ##
==========================================
+ Coverage   72.29%   72.32%   +0.03%     
==========================================
  Files         255      255              
  Lines       12847    12847              
  Branches      817      817              
==========================================
+ Hits         9288     9292       +4     
+ Misses       3559     3555       -4     

see 1 file with indirect coverage changes