Update system text json to non vulnerable version

[Adishone]

Pull Request

The issue or feature being addressed

Used System.Text.Json package is really outdated and has known vulnerabilties

Details on the issue fix or feature implementation

System.Text.Json is known to have vulnerabilities so I'm updating it to version that has them fixed

[martincostello]

Thanks for your PR - this package is specifically pinned to the minimum compatible version because:

1. It's only used for 3 packages, and then only for the netstandard2.0 target framework - other targets will use the in-box version of System.Text.Json, whatever it happens to be for the consuming application.
2. It avoids creating any compatibility issues from a giant leap forward in the version of System.Text.Json (4 major versions, or 5 if we used 9.0.0 instead) used with .NET Framework applications. Testing that such a change doesn't break things is too much of a burden for us to test.
3. We don't want to create churn by having to release a new version of Swashbuckle every time there's a vulnerability in a dependency we happen to use.

Users who aren't otherwise already using a newer patched version in their application's dependency tree can add an explicit reference to a newer version.

You can see which packages we pin here:

Swashbuckle.AspNetCore/.github/dependabot.yml

Lines 42 to 58 in 4ae6541

	ignore:
	# Ignore the libraries which are "pinned"
	- dependency-name: "IdentityServer4"
	- dependency-name: "IdentityServer4.AccessTokenValidation"
	- dependency-name: "Microsoft.AspNetCore"
	- dependency-name: "Microsoft.AspNetCore.App"
	- dependency-name: "Microsoft.AspNetCore.Mvc.ApiExplorer"
	- dependency-name: "Microsoft.AspNetCore.Mvc.DataAnnotations"
	- dependency-name: "Microsoft.AspNetCore.Mvc.Formatters.Json"
	- dependency-name: "Microsoft.AspNetCore.Mvc.NewtonsoftJson"
	- dependency-name: "Microsoft.AspNetCore.Mvc.Testing"
	- dependency-name: "Microsoft.AspNetCore.Routing"
	- dependency-name: "Microsoft.AspNetCore.StaticFiles"
	- dependency-name: "Microsoft.Extensions.FileProviders.Embedded"
	- dependency-name: "Microsoft.OpenApi"
	- dependency-name: "Microsoft.OpenApi.Readers"
	- dependency-name: "System.Text.Json"