Remove the need for experimentalMacroSupport and gate on SDK version #5105
Description
The dart.experimentalMacroSupport setting was used to enable new APIs to get generated code from the server into VS Code. There's been enough testing that this flag is probably unnecessary now, however we should gate enabling these new APIs on a suitable version number where this API support is more stable/complete (perhaps 3.5.0-0, since 3.5 is what current bleeding-edge builds are).