Lock influxdb to 1.8 for now

Relates https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit/-/issues/86

It's hard to tell the specifics as the documentation isn't clear but going by the behavior InfluxDB v2.0.0 hit Docker Hub repos today and it appears to contain various breaking changes including a change to health endpoints and what appears to be the complete removal of the influx execute command.

We're going to stick with the latest v1 release for now to give us time to find out all of these breaking changes and then upgrade.