Fix Experimental -> Beta references
What does this MR do?
Fix Experimental -> Beta references
The instance and Docker Autoscaler executors have changed to Beta since GitLab Runner 16.6: #29404 (closed)
Why was this MR needed?
Because the information currently present on https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runnersinstance-section and https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runnersautoscaler-section is outdated.