postrelease: automatically update the Go version

[zmb3]

Extract the Go version from go.mod and use it when we update the version numbers used throughout the docs.

This has several advantages:

1. We no longer need to remember to periodically update this variable. For example, Teleport 17 was built with Go 1.23 but the docs still (incorrectly) mention Go 1.22.
2. We get even more precision, as we'll start using the full patch version of Go instead of only the minor.