[Feature request]: Replace usage of IdentityServer4 in tests

[martincostello]

Is your feature request related to a specific problem? Or an existing feature?

#3007 flagged that IdentityServer4 is completely deprecated and contains a number of vulnerabilities that cause errors when building with the .NET 9+ SDK (e.g. GHSA-55p7-v223-x366 and GHSA-ff4q-64jc-gx98).

For now, as this is just test code, we can suppress these like this:

Swashbuckle.AspNetCore/test/WebSites/OAuth2Integration/OAuth2Integration.csproj

Lines 18 to 22 in 8114ddd

	<!-- This is just a sample for testing - updating to Duende.IdentityServer can be done at some point in the future -->
	<ItemGroup>
	<NuGetAuditSuppress Include="https://github.com/advisories/GHSA-55p7-v223-x366" />
	<NuGetAuditSuppress Include="https://github.com/advisories/GHSA-ff4q-64jc-gx98" />
	</ItemGroup>

Instead, we should properly fix the errors by using a supported alternative.

Describe the solution you'd like

Replace usage of with a more modern and supported alternative, such as Duende.IdentityServer.

Additional context

No response