Bump Go version to 1.18

[buchdag]

This PR bump Go version to 1.18.

Noteworthy addition are the break and continue actions in text/template (thanks @rhansen for highlighting this):

{{break}}
The innermost {{range pipeline}} loop is ended early, stopping the
current iteration and bypassing all remaining iterations.

{{continue}}
The current iteration of the innermost {{range pipeline}} loop is
stopped, and the loop starts the next iteration.