Support 6.3 snapshot toolchains

[kkebo]

Swift 6.3 snapshot toolchains are now available.

• swiftlang/swift-org-website@0c5950e
• swiftlang/swift-org-website@80aa13f

However, swiftly can't install them since the swift.org API has not yet updated.

• https://github.com/swiftlang/swift-org-website/tree/23a2c61c9dc9b5d47ea3643576117b8933ef68a3/api/v1/install/dev

[kkebo]

• Related issue: API endpoint for 6.3 SDKs swift-org-website#1256

[kkebo]

Closed by swiftlang/swift-org-website#1257