Migrate core deserialization and default JSON library to System.Text.Json

[jar-stripe]

Why?

stripe-dotnet currently uses Newtonsoft.Json as its primary JSON serializer but has carried System.Text.Json (STJ) attributes in parallel since 2024. This PR promotes STJ becomes the primary serializer for all request serialization, response deserialization, and public ToJson() output, and is implemented so that STJ will serialize and deserialize correctly without relying on custom options.

Newtonsoft.Json remains available for backward compatibility (ToJson() previously used Newtonsoft; RawJObject is still populated).

What?

• Remove #if NET6_0_OR_GREATER conditional compilation gates from STJ attributes and adds backfill for STJ on .NET 4.6.2 and .net 5.
• Swap the primary serialization engine from Newtonsoft to STJ in StripeClient, ApiRequestor, and EventUtility
• Create public STJStripeEntityConverter and STJStripeOptionsConverter as JsonConverterFactory classes that every entity/options class references via [JsonConverter]
• Refactor STJDefaultConverter<T> to be the shared read/write implementation used by both factory converters
• Add [JsonExtensionData] support to STJDefaultConverter for ExtraParams on BaseOptions
• Add DecimalStringConverter (Newtonsoft) for decimal_string fields — writes decimals as JSON strings to match STJ's [JsonNumberHandling] behavior
• Add Int64StringConverter (Newtonsoft) for int64_string fields — same pattern as above (cherry-picked from PR Add runtime support for V2 int64 string-encoded fields #3321)
• Add [JsonConverter(typeof(STJEventConverter))] to Event class for pre-2017 event compatibility
• Update ToJson() to use STJ instead of Newtonsoft
• Remove STJMemberSerializationOptIn and related infrastructure (no longer needed with per-type converters)
• Add [JsonConverter(typeof(STJStripeEntityConverter))] to all entity classes
• Add [JsonConverter(typeof(STJStripeOptionsConverter))] to all options classes

This PR also fixes an issue with serializing Stripe's decimal strings

• Add [JsonConverter(typeof(DecimalStringConverter))] on decimal_string Newtonsoft properties

Tests:

• Update tests to primarily use STJ serialization
• Wholesome test NewtonsoftAndSystemTextJsonOutputTheSameObject validates parity between serializers

See Also

• int64_string support: Add runtime support for V2 int64 string-encoded fields #3321

Changelog

• ⚠️ System.Text.Json replaces Newtonsoft Json.NET as the default JSON library used in serialization and deserialization of Stripe.net objects. This is most likely non-breaking for most users.
• ⚠️ Serializing Stripe objects using either System.Text.Json or Newtonsoft Json.NET now represents decimal-format strings as JSON string values to match the Stripe API format.

[jar-stripe]

@mbroshi-stripe I'm working to rebuild the commit history so this is less insane to review

[jar-stripe]

Ok this should be good to review; the first commit is all the manual changes, the second commit is generated code, and then there is some clean up to fix tests after that.