Bump .tool-versions to use Go 1.18.6
The GitLab Development Kit is moving to 1.18.6, so let's make all the versions consistent to reduce the number of interpreters needed for development.
Part of &8843 (closed)
The GitLab Development Kit is moving to 1.18.6, so let's make all the versions consistent to reduce the number of interpreters needed for development.
Part of &8843 (closed)