Upgrade to ruby 2.6-2.7 and fix security issues

• Drop support for ruby < 2.6, tested on 2.6 and 2.7 but may work on later versions. This fixes the SSL errors if you are using NewRelic metrics.
• Migrate to Github actions. Now the docker image is available exclusively on Github ghcr.io/hammady/scaltainer. If you are still using the Dockerub image rayyanqcri/scaltainer, you should consider upgrading as soon as possible.
• Fix vulnerabilities in 2 dependencies, check #5 and #6 for more details.

This introduces New Relic metric reporting.
The following metrics will be reported:

• Custom/WebReplicas/service
• Custom/WorkerReplicas/service
• Custom/WebMetric/service
• Custom/WorkerMetric/service
• Custom/Scaltainer/ticks

Where service is a placeholder for each service defined in your scaltainer.yaml.

To enable New Relic reporting, append --enable-newrelic-reporting to the command line.
Must set NEW_RELIC_LICENSE_KEY and NEW_RELIC_APP_NAME to get it to work.

Here is an example NRQL to query the metrics:

FROM Metric
SELECT max(newrelic.timeslice.value)
WHERE appName = 'YOUR APP NAME'
WITH METRIC_FORMAT 'Custom/WebReplicas/{web}'
FACET web
SINCE 1 day ago TIMESERIES MAX

This introduces new metrics, beside renaming the old ones:

• scaltainer_worker_replicas_total: Same as above, but for workers
• scaltainer_web_response_time_seconds: response times as reported by the web services
• scaltainer_worker_queue_size_total: queue sizes as reported by the worker services
• scaltainer_ticks_total: iterations scaltainer has performed (if -w is used)

Add Prometheus reporting.