Support Go1.21.6

[mboyea]

Describe the bug
Right now, I failed to run the HEAD build with Go1.21.6 while trying to build flyctl on Arch Linux; 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/state
	imports gvisor.dev/gvisor/pkg/state/wire
	imports gvisor.dev/gvisor/pkg/gohacks: build constraints exclude all Go files in /home/mboyea/go/pkg/mod/gvisor.dev/gvisor@v0.0.0-20221203005347-703fd9b7fbc0/pkg/gohacks
make: *** [Makefile:12: build] Error 1

Screenshot

Relevant Issues
#2688

Next Steps
If someone can point me to an upstream issue (like those outlined in this issue), I would be happy to take a look at fixing it, but I have not used Go packages before, so I do not know where to start at this time..

In the meantime I will try to compile with Go1.20 or wait for Go1.22

[dangra]

the AUR package is outdated, it is pointing to flyctl v0.1.27 which is extremely old. Consider installing https://aur.archlinux.org/packages/flyctl-bin which is kept up to date.

[mboyea]

@dangra Thank you for the help, apologies for my negligence.