Remove support for non-utf8 strings (SYN-8861)

[Cisphyx]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 98.52941% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.39%. Comparing base (7c22c5b) to head (c4350c8).
Report is 58 commits behind head on synapse-3xx.

Files with missing lines	Patch %	Lines
synapse/lib/msgpack.py	97.14%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           synapse-3xx    #4191      +/-   ##
===============================================
+ Coverage        96.38%   96.39%   +0.01%     
===============================================
  Files              247      247              
  Lines            57809    57815       +6     
===============================================
+ Hits             55717    55731      +14     
+ Misses            2092     2084       -8     

Flag	Coverage Δ
linux	96.39% <98.52%> (+0.01%)	⬆️

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