Update toolchain and code to Go 1.22

[rg0now]

The default behavior of for variables has changed in Go 1.22. This issue should track the progress in updating everything (see below) to Go 1.22.

Previously, the variables declared by a "for" loop were created once and updated by each iteration. In Go 1.22, each iteration of the loop creates new variables, to avoid accidental sharing bugs. This removes a number of stupid GOTCHAs.

Proposed changes:
☐ update the Go toolchain in the install scripts and the VM
☐ update slides: remove all occurrences of protecting against the "sharing" GOTCHA, including potentially the cases when we create new goroutines in the for loop (check this)
☐ update sample code that comes with the slide-deck
☐ update labs