Updates swagger-ui submodule to v3.20.9

[curiousdev]

Updates swagger-ui submodule to v3.20.9

[curiousdev]

@domaindrivendev I'm reviewing the failing test below. Some assets are no longer included. Am I overlooking something?

Swashbuckle.Tests.SwaggerUi.SwaggerUiTests.It_returns_correct_asset_mime_type("http://tempuri.org/swagger/ui/css/typography-css","text/css")
[OK] http://tempuri.org/swagger/ui/css/typography-css => text/css
[OK] http://tempuri.org/swagger/ui/css/typography-css => text/css
=> Swashbuckle.Tests.SwaggerUi.SwaggerUiTests.It_returns_correct_asset_mime_type("http://tempuri.org/swagger/ui/images/logo_small-png","image/png")
[NotFound] http://tempuri.org/swagger/ui/images/logo_small-png => application/json
[NotFound] http://tempuri.org/swagger/ui/images/logo_small-png => application/json

[curiousdev]

@domaindrivendev Are there remaining open items for the PR?

[M0ns1gn0r]

@domaindrivendev could you please check this PR, everyone wants the new UI :)

[winter3angle]

@domaindrivendev kindly take a look at this, there are some annoying bugs in ancient swagger-ui, like #1253