What are the supported Swift compiler versions to build and test Testing from the main branch?

### Description I looked through the doc and issues and didn't find this specified anywhere. Normal Swift packages can support multiple Swift compiler versions, but toolchain repos like this are usually only built with the latest trunk compiler also, or the matching release/6.3 compiler for the release/6.3 branch here. However, I see that the main branch here has several actions building it with the Swift 6.3 compiler also, so I wonder if this is a planned change in version support or something that has just crept in. Either way, it would be good to explicitly specify the planned version support for Testing.