[sg] Upgrade urfave/cli from v2 to v3

[jamesmcnamara]

This seems like it will fix the bash completion problem but we are blocked until urfave/cli#833 is released and its outstanding issues are fixed. Current known issues:

• v3 hardcodes the bash-completion parameter --generate-shell-completion even though they have documented that it is customizable
• Autocompletions do not have access to flags and don't currently autocomplete flags or offer flag sensitive values
• background.loadFromContext throws some error when trying to cast one type to another:

goroutine 1 [running]:
github.com/sourcegraph/sourcegraph/dev/sg/internal/background.loadFromContext(...)
	dev/sg/internal/background/background.go:40
github.com/sourcegraph/sourcegraph/dev/sg/internal/background.Wait({0x1097245d8, 0x14002912a50}, 0x14002b012e0)
	dev/sg/internal/background/background.go:70 +0x5d4
main.init.func54({0x1097245d8, 0x14002912a50}, 0x1?)
	dev/sg/main.go:253 +0x40
github.com/urfave/cli/v3.(*Command).Run.func1()
	external/com_github_urfave_cli_v3/command.go:532 +0x48
github.com/urfave/cli/v3.(*Command).Run(0x10bd00e40, {0x1097244d8, 0x10ea8bee0}, {0x140001ca000, 0x5, 0x5})
	external/com_github_urfave_cli_v3/command.go:620 +0x12dc
main.main()
	dev/sg/main.go:36 +0xa0

Test plan

Ran some tests locally but this needs more thorough testing.

[bobheadxi]

FYI related: https://github.com/sourcegraph/devx-support/issues/423