Tailscale package is very old

The package listed in the opkg source is a very old version for all architectures https://downloads.openwrt.org/releases/23.05.3/packages/

Who do I need to contact to ask them to update it?

I think their requirements exceed go 1.21 in OpenWRT 23.05.

The nature of stable releases is that packages aren't getting updated to new major versions (apart from security- and bug fixes), so unless there is a strong need for an update, it won't be. Yes, the circumstances are a bit more relaxed for leaf packages in the package feeds compared to essential packages, but the same principles more or less apply.