Switch to Go 1.21 for official builds

[mx-psi]

Same as #286, but for Go 1.21.

Go 1.21 was released recently: https://go.dev/doc/go1.21

After we add testing for it on core (open-telemetry/opentelemetry-collector/issues/8207) and contrib (open-telemetry/opentelemetry-collector-contrib/issues/25116) we can switch to using the Go 1.21 compiler for official builds. Based on historical precedent this will make our life easier when dealing with security issues since we don't depend on the Go team backporting fixes and can react more quickly.

[rmfitzpatrick]

Do the eol constraints for windows and darwin impact this decision?

Windows
Go 1.20 is the last release that will run on any release of Windows 7, 8, Server 2008 and Server 2012. Go 1.21 will require at least Windows 10 or Server 2016.

Darwin and iOS
Go 1.20 is the last release that will run on macOS 10.13 High Sierra or 10.14 Mojave. Go 1.21 will require macOS 10.15 Catalina or later.

https://tip.golang.org/doc/go1.20

[mx-psi]

As mentioned on the Collector SIG, we are working on open-telemetry/opentelemetry-collector/issues/8209 to define the support we give to each platform. I think it would be a good place to discuss and define this.