update package references to github.com/spinframework/spin-sdk-go

[bacongobbler]

Given a bare repository:

><> mkdir foo && cd foo
><> go mod init example.com/m

This works:

><> go get github.com/fermyon/spin-go-sdk
go: downloading github.com/fermyon/spin-go-sdk v0.0.0-20250310233821-2665ad7e30a4
go: added github.com/fermyon/spin-go-sdk v0.0.0-20250310233821-2665ad7e30a4

This does not:

><> go get github.com/spinframework/spin-go-sdk
go: github.com/spinframework/spin-go-sdk@upgrade (v0.0.0-20250310233821-2665ad7e30a4) requires github.com/spinframework/spin-go-sdk@v0.0.0-20250310233821-2665ad7e30a4: parsing go.mod:
        module declares its path as: github.com/fermyon/spin-go-sdk
                but was required as: github.com/spinframework/spin-go-sdk

I'm assuming all we need to change is this line and update all the examples. The existing package name should continue to work for existing apps because the SHA referenced in the package version should point to the correct module name, but apps will probably need to migrate if they run go get -u.