Dialog: Fix possible deadlock

[ScarletKuro]

Description

Possible fix to #8961?
According to Async Guidance it always preferable to create TaskCompletionSource with TaskCreationOptions.RunContinuationsAsynchronously overload: https://github.com/davidfowl/AspNetCoreDiagnosticScenarios/blob/master/AsyncGuidance.md#always-create-taskcompletionsourcet-with-taskcreationoptionsruncontinuationsasynchronously
Also similar thing was done in Blazored/Modal: Blazored/Modal#408

How Has This Been Tested?

Would be nice to have a bUnit test, but I don't have any example that runs into that problem.

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 90.48%. Comparing base (28bc599) to head (d612622).
Report is 193 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8964      +/-   ##
==========================================
+ Coverage   89.82%   90.48%   +0.65%     
==========================================
  Files         412      419       +7     
  Lines       11878    12211     +333     
  Branches     2364     2385      +21     
==========================================
+ Hits        10670    11049     +379     
+ Misses        681      627      -54     
- Partials      527      535       +8     

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