Update MediatR to 9.0.0

[chrissimon-au]

A possible fix for #676

Sets the MediatR version using the version range syntax to [9.0.0,10.0.0) (as a precaution for future breaking changes in MediatR v10.

Have done a local build and all tests pass without any changes. It seems that just building against the explicit 9.0.0 dependency is sufficient to ensure the correct mediator interface method is invoked.

[codecov[bot]]

Codecov Report

Merging #678 (0647488) into master (112f240) will decrease coverage by 3.57%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #678      +/-   ##
==========================================
- Coverage   73.04%   69.47%   -3.58%     
==========================================
  Files         260      260              
  Lines       12624    12624              
  Branches      853      853              
==========================================
- Hits         9221     8770     -451     
- Misses       3403     3580     +177     
- Partials        0      274     +274     

Impacted Files	Coverage Δ
src/JsonRpc/ExternalServiceProvider.cs	50.00% <0.00%> (-50.00%)	:arrow_down:
src/JsonRpc.Generators/SymbolExtensions.cs	71.42% <0.00%> (-28.58%)	:arrow_down:
src/JsonRpc/Server/ContentModifiedException.cs	37.50% <0.00%> (-18.75%)	:arrow_down:
src/JsonRpc/Server/RequestCancelledException.cs	37.50% <0.00%> (-18.75%)	:arrow_down:
...rver/Configuration/ChainedConfigurationProvider.cs	58.62% <0.00%> (-17.25%)	:arrow_down:
...ient/Configuration/ChainedConfigurationProvider.cs	0.00% <0.00%> (-17.25%)	:arrow_down:
...rver/Configuration/ChainedConfigurationProvider.cs	0.00% <0.00%> (-17.25%)	:arrow_down:
src/JsonRpc/Server/ServerErrorResult.cs	83.33% <0.00%> (-16.67%)	:arrow_down:
src/JsonRpc.Generators/Contexts/SyntaxSymbol.cs	75.00% <0.00%> (-16.67%)	:arrow_down:
src/Testing/PositionMarker.cs	21.05% <0.00%> (-15.79%)	:arrow_down:
... and 98 more

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[JoeRobich]

Took update in https://github.com/OmniSharp/csharp-language-server-protocol/pull/1084