Support go1.21.0

[chenrui333]

Please only report specific issues with flyctl behavior. Anything like a support request for your application should go to https://community.fly.io. More people watch that space and can help you faster!

Describe the bug
Right now, I failed to run the HEAD build, the error is shown below:

package github.com/superfly/flyctl
	imports github.com/superfly/flyctl/internal/cli
	imports github.com/superfly/flyctl/internal/command/root
	imports github.com/superfly/flyctl/internal/command
	imports github.com/superfly/flyctl/internal/appconfig
	imports github.com/superfly/flyctl/flaps
	imports github.com/superfly/flyctl/agent
	imports github.com/superfly/flyctl/internal/wireguard
	imports github.com/superfly/flyctl/wg
	imports golang.zx2c4.com/wireguard/tun/netstack
	imports gvisor.dev/gvisor/pkg/bufferv2
	imports gvisor.dev/gvisor/pkg/atomicbitops
	imports gvisor.dev/gvisor/pkg/cpuid
	imports gvisor.dev/gvisor/pkg/state
	imports gvisor.dev/gvisor/pkg/state/wire
	imports gvisor.dev/gvisor/pkg/gohacks: build constraints exclude all Go files in /Users/rui/go/pkg/mod/gvisor.dev/[email protected]/pkg/gohacks

relates to Homebrew/homebrew-core#134468

[dangra]

For reference this is holding on this change WireGuard/wireguard-go#86.