Build official releases with Go 1.18

There is no special reason to do this; we just typically adopt the latest
minor release of the Go toolchain for each new minor release of
Terraform CLI so that we can make use of its new library and language
features gradually over the subsequent patch releases.

Adopting early will give us more time to exercise this and catch any
wrinkles before the Terraform CLI v1.2 release.

.go-version

@@ -1 +1 @@
-1.17.8
+1.18.0